gh-36898: ruff auto-fixing several details in combinat folder
    
this was scripted using `ruff --fix
--select=UP009,UP018,UP034,UP039,UP025 src/sage/combinat`

see https://docs.astral.sh/ruff/rules/#pyupgrade-up

### 📝 Checklist

- [x] The title is concise, informative, and self-explanatory.
- [x] The description explains in detail what this PR is about.
    
URL: #36898
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert

Author: Release Manager

src/sage/combinat/abstract_tree.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 r"""
 Abstract Recursive Trees
 
@@ -73,7 +72,7 @@
 # of it later.
 
 
-class AbstractTree():
+class AbstractTree:
     """
     Abstract Tree.
 
@@ -1276,7 +1275,7 @@ def node_to_str(t):
             if hasattr(t, "label"):
                 return str(t.label())
             else:
-                return u"o"
+                return "o"
         # other possible choices for nodes would be u"█ ▓ ░ ╋ ╬"
 
         if self.is_empty():
@@ -1291,9 +1290,9 @@ def node_to_str(t):
 
         if len(self) == 1:
             repr_child = self[0]._unicode_art_()
-            sep = UnicodeArt([u" " * repr_child._root])
+            sep = UnicodeArt([" " * repr_child._root])
             t_repr = UnicodeArt([node_to_str(self)])
-            repr_root = (sep + t_repr) * (sep + UnicodeArt([u"│"]))
+            repr_root = (sep + t_repr) * (sep + UnicodeArt(["│"]))
             t_repr = repr_root * repr_child
             t_repr._root = repr_child._root
             t_repr._baseline = t_repr._h - 1
@@ -1303,17 +1302,17 @@ def node_to_str(t):
         l_repr = [subtree._unicode_art_() for subtree in self]
         acc = l_repr.pop(0)
         whitesep = acc._root
-        lf_sep = u" " * whitesep + u"╭" + u"─" * (acc._l - acc._root)
-        ls_sep = u" " * whitesep + u"│" + u" " * (acc._l - acc._root)
+        lf_sep = " " * whitesep + "╭" + "─" * (acc._l - acc._root)
+        ls_sep = " " * whitesep + "│" + " " * (acc._l - acc._root)
         while l_repr:
             tr = l_repr.pop(0)
-            acc += UnicodeArt([u" "]) + tr
+            acc += UnicodeArt([" "]) + tr
             if not len(l_repr):
-                lf_sep += u"─" * (tr._root) + u"╮"
-                ls_sep += u" " * (tr._root) + u"│"
+                lf_sep += "─" * (tr._root) + "╮"
+                ls_sep += " " * (tr._root) + "│"
             else:
-                lf_sep += u"─" * (tr._root) + u"┬" + u"─" * (tr._l - tr._root)
-                ls_sep += u" " * (tr._root) + u"│" + u" " * (tr._l - tr._root)
+                lf_sep += "─" * (tr._root) + "┬" + "─" * (tr._l - tr._root)
+                ls_sep += " " * (tr._root) + "│" + " " * (tr._l - tr._root)
         mid = whitesep + (len(lf_sep) - whitesep) // 2
         node = node_to_str(self)
         lf_sep = (lf_sep[:mid - len(node) // 2] + node +
@@ -1501,7 +1500,7 @@ def create_node(self):
                   . the matrix
                   . and the edges
                 """
-                name = "".join((chr(ord(x) + 49) for x in str(num[0])))
+                name = "".join(chr(ord(x) + 49) for x in str(num[0]))
                 node = cmd + name
                 nodes.append((name,
                     (str(self.label()) if hasattr(self, "label") else ""))

src/sage/combinat/backtrack.py

@@ -36,7 +36,7 @@
 from sage.sets.recursively_enumerated_set import RecursivelyEnumeratedSet_forest
 
 
-class GenericBacktracker():
+class GenericBacktracker:
     r"""
     A generic backtrack tool for exploring a search space organized as a tree,
     with branch pruning, etc.

src/sage/combinat/baxter_permutations.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 """
 Baxter permutations
 """

src/sage/combinat/bijectionist.py

@@ -2455,7 +2455,7 @@ def solutions_iterator(self):
         yield from self._bmilp.solutions_iterator(False, [])
 
 
-class _BijectionistMILP():
+class _BijectionistMILP:
     r"""
     Wrapper class for the MixedIntegerLinearProgram (MILP).
 

src/sage/combinat/binary_tree.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 """
 Binary Trees
 
@@ -593,7 +592,7 @@ def _unicode_art_(self):
                   o   o   o   o
         """
         def node_to_str(bt):
-            return str(bt.label()) if hasattr(bt, "label") else u"o"
+            return str(bt.label()) if hasattr(bt, "label") else "o"
 
         if self.is_empty():
             from sage.typeset.unicode_art import empty_unicode_art
@@ -609,14 +608,14 @@ def node_to_str(bt):
             node = node_to_str(self)
             rr_tree = self[1]._unicode_art_()
             if rr_tree._root > 2:
-                f_line = u" " * (rr_tree._root - 3) + node
-                s_line = u" " * (len(node) + rr_tree._root - 3) + u"╲"
+                f_line = " " * (rr_tree._root - 3) + node
+                s_line = " " * (len(node) + rr_tree._root - 3) + "╲"
                 t_repr = UnicodeArt([f_line, s_line]) * rr_tree
                 t_repr._root = rr_tree._root - 2
             else:
                 f_line = node
-                s_line = u" ╲"
-                t_line = u" " * (len(node) + 1)
+                s_line = " ╲"
+                t_line = " " * (len(node) + 1)
                 t_repr = UnicodeArt([f_line, s_line]) * (UnicodeArt([t_line]) + rr_tree)
                 t_repr._root = rr_tree._root
             t_repr._baseline = t_repr._h - 1
@@ -625,8 +624,8 @@ def node_to_str(bt):
         if self[1].is_empty():
             node = node_to_str(self)
             lr_tree = self[0]._unicode_art_()
-            f_line = u" " * (lr_tree._root + 1) + node
-            s_line = u" " * lr_tree._root + u"╱"
+            f_line = " " * (lr_tree._root + 1) + node
+            s_line = " " * lr_tree._root + "╱"
             t_repr = UnicodeArt([f_line, s_line]) * lr_tree
             t_repr._root = lr_tree._root + 2
             t_repr._baseline = t_repr._h - 1
@@ -638,10 +637,10 @@ def node_to_str(bt):
         nb_ = lr_tree._l - lr_tree._root + rr_tree._root - 1
         nb_L = nb_ // 2
         nb_R = nb_L + (nb_ % 2)
-        f_line = u" " * (lr_tree._root + 1) + u"_" * nb_L + node
-        f_line += u"_" * nb_R
-        s_line = u" " * lr_tree._root + u"╱" + u" " * (len(node) + rr_tree._root - 1 + (lr_tree._l - lr_tree._root)) + u"╲"
-        t_repr = UnicodeArt([f_line, s_line]) * (lr_tree + UnicodeArt([u" " * (len(node) + 2)]) + rr_tree)
+        f_line = " " * (lr_tree._root + 1) + "_" * nb_L + node
+        f_line += "_" * nb_R
+        s_line = " " * lr_tree._root + "╱" + " " * (len(node) + rr_tree._root - 1 + (lr_tree._l - lr_tree._root)) + "╲"
+        t_repr = UnicodeArt([f_line, s_line]) * (lr_tree + UnicodeArt([" " * (len(node) + 2)]) + rr_tree)
         t_repr._root = lr_tree._root + nb_L + 2
         t_repr._baseline = t_repr._h - 1
         return t_repr

src/sage/combinat/combinatorial_map.py

@@ -197,7 +197,7 @@ def combinatorial_map_wrapper(f=None, order=None, name=None):
 # combinatorial_map = combinatorial_map_wrapper
 
 
-class CombinatorialMap():
+class CombinatorialMap:
     r"""
     This is a wrapper class for methods that are *combinatorial maps*.
 

src/sage/combinat/composition.py

@@ -1,4 +1,3 @@
-# -*- coding: utf-8 -*-
 r"""
 Integer compositions
 

src/sage/combinat/composition_tableau.py

@@ -120,7 +120,7 @@ def __init__(self, parent, t):
 
         CombinatorialElement.__init__(self, parent, t)
 
-    def _repr_diagram(self):
+    def _repr_diagram(self) -> str:
         r"""
         Return a string representation of ``self`` as an array.
 
@@ -132,8 +132,8 @@ def _repr_diagram(self):
               3  2
               4  4
         """
-        return '\n'.join(("".join(("%3s" % str(x) for x in row))
-                          for row in self))
+        return '\n'.join("".join("%3s" % str(x) for x in row)
+                         for row in self)
 
     def __call__(self, *cell):
         r"""

src/sage/combinat/crystals/generalized_young_walls.py

@@ -285,7 +285,7 @@ def generate_signature(self, i):
         strsig = ''.join( x[0] for x in sig)
         reducedsig = strsig
         while re.search(r"\+\s*-",reducedsig):
-            reducedsig = re.sub(r"\+\s*-", lambda match : str().ljust(len(match.group(int(0)))) , reducedsig)
+            reducedsig = re.sub(r"\+\s*-", lambda match : ''.ljust(len(match.group(0))) , reducedsig)
         return (sig,reducedsig)
 
     def signature(self, i):

src/sage/combinat/crystals/tensor_product.py

@@ -655,7 +655,7 @@ class Element(TensorProductOfSuperCrystalsElement):
         pass
 
 
-class QueerSuperCrystalsMixin():
+class QueerSuperCrystalsMixin:
     """
     Mixin class with methods for a finite queer supercrystal.
     """