gh-36278: `sage.graphs`: some care with return ... else statements in some .pyx files
    
We change this kind of statements in files `src/sage/graphs/*.pyx`:
```py
if ....:
    return ...
else:
    return ...
```
to
```py
if ....:
    return ...
return ...
```

### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->
<!-- If your change requires a documentation PR, please link it
appropriately -->
<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
<!-- Feel free to remove irrelevant items. -->

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on
- #12345: short description why this is a dependency
- #34567: ...
-->

<!-- If you're unsure about any of these, don't hesitate to ask. We're
here to help! -->
    
URL: #36278
Reported by: David Coudert
Reviewer(s): github-actions[bot], Kwankyu Lee

Author: Release Manager

src/sage/graphs/bliss.pyx

@@ -375,8 +375,7 @@ cdef canonical_form_from_edge_list(int Vnr, list Vout, list Vin, int Lnr=1, list
 
     if certificate:
         return new_edges, relabel
-    else:
-        return new_edges
+    return new_edges
 
 
 cpdef canonical_form(G, partition=None, return_graph=False, use_edge_labels=True, certificate=False):

src/sage/graphs/centrality.pyx

@@ -119,8 +119,7 @@ def centrality_betweenness(G, bint exact=False, bint normalize=True):
     """
     if exact:
         return centrality_betweenness_C(G, <mpq_t> 0, normalize=normalize)
-    else:
-        return centrality_betweenness_C(G, <double>0, normalize=normalize)
+    return centrality_betweenness_C(G, <double>0, normalize=normalize)
 
 
 @cython.cdivision(True)

src/sage/graphs/comparability.pyx

@@ -306,15 +306,13 @@ def greedy_is_comparability(g, no_certificate=False, equivalence_class=False):
             # added twice.
             return True, sorted(set(edges))
 
-        else:
-            return True
-    else:
-        if no_certificate:
-            certif.append(certif[0])
-            cycle = [v for v, _ in certif]
-            return False, cycle
-        else:
-            return False
+        return True
+
+    if no_certificate:
+        certif.append(certif[0])
+        cycle = [v for v, _ in certif]
+        return False, cycle
+    return False
 
 
 def greedy_is_comparability_with_certificate(g, certificate=False):
@@ -361,8 +359,7 @@ def greedy_is_comparability_with_certificate(g, certificate=False):
     if not isit:
         if certificate:
             return False, certif
-        else:
-            return False
+        return False
 
     elif not certificate:
         return True
@@ -552,8 +549,7 @@ def is_comparability(g, algorithm="greedy", certificate=False, check=True,
         if certificate:
             from sage.graphs.digraph import DiGraph
             return True, DiGraph(g)
-        else:
-            return True
+        return True
 
     if algorithm == "greedy":
         comparability_test = greedy_is_comparability_with_certificate(g, certificate=certificate)
@@ -677,46 +673,44 @@ def is_permutation(g, algorithm="greedy", certificate=False, check=True,
        ....:        break
 
     """
-    if certificate:
-
-        # First poset, we stop if it fails
-        isit, certif = is_comparability(g, algorithm=algorithm, certificate=True,
-                                        solver=solver, verbose=verbose)
-        if not isit:
-            return False, certif
-
-        # Second poset
-        isit, co_certif = is_comparability(g.complement(), algorithm=algorithm, certificate=True,
-                                           solver=solver, verbose=verbose)
-        if not isit:
-            return False, co_certif
-
-        # Building the two orderings
-        tmp = list(co_certif.edges(labels=False, sort=False))
-        for u, v in certif.edge_iterator(labels=False):
-            co_certif.add_edge(v, u)
-        certif.add_edges(tmp)
-
-        ordering = certif.topological_sort()
-        co_ordering = co_certif.topological_sort()
-
-        # Try to build the Permutation graph from the permutations, just to make
-        # sure nothing weird happened !
-        if check:
-            from sage.graphs.graph_generators import GraphGenerators
-            pg = GraphGenerators().PermutationGraph(ordering, co_ordering)
-            if not pg.is_isomorphic(g):
-                raise ValueError("There is a mistake somewhere ! It looks like "
-                                 "the Permutation Graph model computed does "
-                                 "not match the input graph !")
-
-        return True, (ordering, co_ordering)
-
-    # No certificate... A piece of cake
-    else:
+    if not certificate:
+        # No certificate... A piece of cake
         return (is_comparability(g, algorithm=algorithm, solver=solver, verbose=verbose) and
                 is_comparability(g.complement(), algorithm=algorithm, solver=solver, verbose=verbose))
 
+    # First poset, we stop if it fails
+    isit, certif = is_comparability(g, algorithm=algorithm, certificate=True,
+                                    solver=solver, verbose=verbose)
+    if not isit:
+        return False, certif
+
+    # Second poset
+    isit, co_certif = is_comparability(g.complement(), algorithm=algorithm, certificate=True,
+                                       solver=solver, verbose=verbose)
+    if not isit:
+        return False, co_certif
+
+    # Building the two orderings
+    tmp = list(co_certif.edges(labels=False, sort=False))
+    for u, v in certif.edge_iterator(labels=False):
+        co_certif.add_edge(v, u)
+    certif.add_edges(tmp)
+
+    ordering = certif.topological_sort()
+    co_ordering = co_certif.topological_sort()
+
+    # Try to build the Permutation graph from the permutations, just to make
+    # sure nothing weird happened !
+    if check:
+        from sage.graphs.graph_generators import GraphGenerators
+        pg = GraphGenerators().PermutationGraph(ordering, co_ordering)
+        if not pg.is_isomorphic(g):
+            raise ValueError("There is a mistake somewhere ! It looks like "
+                             "the Permutation Graph model computed does "
+                             "not match the input graph !")
+
+    return True, (ordering, co_ordering)
+
 
 def is_transitive(g, certificate=False):
     r"""

src/sage/graphs/connectivity.pyx

@@ -1177,8 +1177,7 @@ def edge_connectivity(G,
             return 0
         elif vertices:
             return [0, [], [{}, {}]]
-        else:
-            return [0, []]
+        return [0, []]
 
     if implementation == "boost":
         from sage.graphs.base.boost_graph import edge_connectivity
@@ -1275,29 +1274,27 @@ def edge_connectivity(G,
     if value_only:
         return obj
 
-    else:
-        val = [obj]
-
-        in_set = p.get_values(in_set, convert=bool, tolerance=integrality_tolerance)
+    val = [obj]
+    in_set = p.get_values(in_set, convert=bool, tolerance=integrality_tolerance)
 
-        if g.is_directed():
-            edges = [(u, v, l) for u, v, l in g.edge_iterator() if in_cut[u, v]]
-        else:
-            edges = [(u, v, l) for u, v, l in g.edge_iterator() if in_cut[frozenset((u, v))]]
+    if g.is_directed():
+        edges = [(u, v, l) for u, v, l in g.edge_iterator() if in_cut[u, v]]
+    else:
+        edges = [(u, v, l) for u, v, l in g.edge_iterator() if in_cut[frozenset((u, v))]]
 
-        val.append(edges)
+    val.append(edges)
 
-        if vertices:
-            a = {}
-            b = {}
-            for v in g:
-                if in_set[0, v]:
-                    a.add(v)
-                else:
-                    b.add(v)
-            val.append([a, b])
+    if vertices:
+        a = {}
+        b = {}
+        for v in g:
+            if in_set[0, v]:
+                a.add(v)
+            else:
+                b.add(v)
+        val.append([a, b])
 
-        return val
+    return val
 
 
 def vertex_connectivity(G, value_only=True, sets=False, k=None, solver=None, verbose=0,
@@ -1503,8 +1500,7 @@ def vertex_connectivity(G, value_only=True, sets=False, k=None, solver=None, ver
             return max(g.order() - 1, 0)
         elif not sets:
             return max(g.order() - 1, 0), []
-        else:
-            return max(g.order() - 1, 0), [], [[], []]
+        return max(g.order() - 1, 0), [], [[], []]
 
     if value_only:
         if G.is_directed():
@@ -3298,8 +3294,7 @@ cdef class TriconnectivitySPQR:
         cdef _LinkedListNode * head = _LinkedList_get_head(self.highpt[v])
         if head:
             return head.data
-        else:
-            return 0
+        return 0
 
     cdef __del_high(self, int e_index):
         """

src/sage/graphs/distances_all_pairs.pyx

@@ -629,8 +629,7 @@ def is_distance_regular(G, parameters=False):
         bi[diameter] = None
         ci[0] = None
         return bi, ci
-    else:
-        return True
+    return True
 
 
 ###################################
@@ -1882,8 +1881,7 @@ def diameter(G, algorithm=None, source=None):
     if LB < 0 or LB > n:
         from sage.rings.infinity import Infinity
         return +Infinity
-    else:
-        return int(LB)
+    return int(LB)
 
 
 ###########
@@ -2722,8 +2720,7 @@ def floyd_warshall(gg, paths=True, distances=False):
     if not gverts:
         if distances and paths:
             return {}, {}
-        else:
-            return {}
+        return {}
 
     cdef unsigned int n = max(gverts) + 1
 

src/sage/graphs/generic_graph_pyx.pyx

@@ -487,11 +487,10 @@ def small_integer_to_graph6(n):
     """
     if n < 63:
         return chr(n + 63)
-    else:
-        # get 18-bit rep of n
-        n = int_to_binary_string(n)
-        n = '0'*(18 - len(n)) + n
-        return chr(126) + binary_string_to_graph6(n)
+    # get 18-bit rep of n
+    n = int_to_binary_string(n)
+    n = '0'*(18 - len(n)) + n
+    return chr(126) + binary_string_to_graph6(n)
 
 
 def length_and_string_from_graph6(s):

src/sage/graphs/genus.pyx

@@ -597,31 +597,31 @@ def simple_connected_graph_genus(G, set_embedding=False, check=True, minimal=Tru
 
     if minimal and G.is_planar(set_embedding=set_embedding):
         return 0
+
+    if check:
+        if not G.is_connected():
+            raise ValueError("Cannot compute the genus of a disconnected graph")
+
+        if G.is_directed() or G.has_multiple_edges() or G.has_loops():
+            G = G.to_simple()
+
+    G, vmap = G.relabel(inplace=False, return_map=True)
+    backmap = {u: v for v, u in vmap.items()}
+    G = Graph(G, sparse=False)
+    GG = simple_connected_genus_backtracker(G._backend.c_graph()[0])
+
+    if minimal:
+        style = 1
+        cutoff = 1
     else:
-        if check:
-            if not G.is_connected():
-                raise ValueError("Cannot compute the genus of a disconnected graph")
-
-            if G.is_directed() or G.has_multiple_edges() or G.has_loops():
-                G = G.to_simple()
-
-        G, vmap = G.relabel(inplace=False, return_map=True)
-        backmap = {u: v for v, u in vmap.items()}
-        G = Graph(G, sparse=False)
-        GG = simple_connected_genus_backtracker(G._backend.c_graph()[0])
-
-        if minimal:
-            style = 1
-            cutoff = 1
-        else:
-            style = 2
-            cutoff = 1 + (G.num_edges() - G.num_verts()) / 2  # rounding here is ok
-
-        g = GG.genus(style=style, cutoff=cutoff, record_embedding=set_embedding)
-        if set_embedding:
-            oE = {}
-            E = GG.get_embedding()
-            for v in E:
-                oE[backmap[v]] = [backmap[x] for x in E[v]]
-            oG.set_embedding(oE)
-        return g
+        style = 2
+        cutoff = 1 + (G.num_edges() - G.num_verts()) / 2  # rounding here is ok
+
+    g = GG.genus(style=style, cutoff=cutoff, record_embedding=set_embedding)
+    if set_embedding:
+        oE = {}
+        E = GG.get_embedding()
+        for v in E:
+            oE[backmap[v]] = [backmap[x] for x in E[v]]
+        oG.set_embedding(oE)
+    return g

src/sage/graphs/graph_coloring.pyx

@@ -555,8 +555,7 @@ def vertex_coloring(g, k=None, value_only=False, hex_colors=False, solver=None,
                 return 0
             elif hex_colors:
                 return dict()
-            else:
-                return []
+            return []
         # - Independent set
         if not g.size():
             if value_only:
@@ -1754,11 +1753,10 @@ def round_robin(n):
             g.set_edge_label(n - 1, i, i)
             for j in range(1, (n - 1) // 2 + 1):
                 g.set_edge_label(my_mod(i - j, n - 1), my_mod(i + j, n - 1), i)
-        return g
     else:
         g = round_robin(n + 1)
         g.delete_vertex(n)
-        return g
+    return g
 
 
 def linear_arboricity(g, plus_one=None, hex_colors=False, value_only=False,

src/sage/graphs/hyperbolicity.pyx

@@ -327,8 +327,7 @@ cdef tuple hyperbolicity_basic_algorithm(int N,
     # Last, we return the computed value and the certificate
     if h_LB != -1:
         return (h_LB, certificate)
-    else:
-        return (-1, [])
+    return (-1, [])
 
 
 ######################################################################
@@ -825,10 +824,10 @@ cdef tuple hyperbolicity_BCCM(int N,
     # Last, we return the computed value and the certificate
     if not certificate:
         return (-1, [], h_UB)
-    else:
-        # When using far-apart pairs, the loops may end before improving the
-        # upper-bound
-        return (h, certificate, h_UB)
+
+    # When using far-apart pairs, the loops may end before improving the
+    # upper-bound
+    return (h, certificate, h_UB)
 
 
 ######################################################################
@@ -1043,10 +1042,10 @@ cdef tuple hyperbolicity_CCL(int N,
     # Last, we return the computed value and the certificate
     if not certificate:
         return (-1, [], h_UB)
-    else:
-        # When using far-apart pairs, the loops may end before improving the
-        # upper-bound
-        return (h, certificate, h_UB if GOTO_RETURN else h)
+
+    # When using far-apart pairs, the loops may end before improving the
+    # upper-bound
+    return (h, certificate, h_UB if GOTO_RETURN else h)
 
 
 def hyperbolicity(G,