gh-36142: python-style loops in `sage.graphs`
    
Convert the loops of the form `for x from 0 <= x < n` in `sage.graphs`
to python-style loops.

### 📝 Checklist

- [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: #36142
Reported by: David Coudert
Reviewer(s): David Coudert, Frédéric Chapoton

Author: Release Manager

src/sage/graphs/chrompoly.pyx

@@ -185,7 +185,7 @@ def chromatic_polynomial(G, return_tree_basis=False, algorithm='C', cache=None):
     # Breadth first search from 0:
     bfs_reorder[0] = 0
     mpz_init(tot[0])  # sets to 0
-    for i from 0 < i < nverts:
+    for i in range(1, nverts):
         bfs_reorder[i] = -1
         mpz_init(tot[i])  # sets to 0
     mpz_init(tot[nverts])  # sets to 0
@@ -230,12 +230,12 @@ def chromatic_polynomial(G, return_tree_basis=False, algorithm='C', cache=None):
         for i in range(nverts):
             mpz_clear(tot[i])
         raise
-    for i from 0 <= i <= nverts:
+    for i in range(nverts + 1):
         mpz_init(coeffs[i])  # also sets them to 0
     mpz_init(coeff)
     mpz_init_set_si(m, -1)
     # start with the zero polynomial: f(x) = 0
-    for i from nverts >= i > 0:
+    for i in range(nverts, 0, -1):  # nverts >= i > 0
         if not mpz_sgn(tot[i]):
             continue
         mpz_neg(m, m)
@@ -244,7 +244,7 @@ def chromatic_polynomial(G, return_tree_basis=False, algorithm='C', cache=None):
         # f += tot[i]*m*x*(x-1)**(i-1)
         mpz_addmul(coeffs[i], m, tot[i])
         mpz_set_si(coeff, 1)
-        for j from 1 <= j < i:
+        for j in range(1, i):
             # an iterative method for binomial coefficients...
             mpz_mul_si(coeff, coeff, j-i)
             mpz_divexact_ui(coeff, coeff, j)
@@ -254,13 +254,13 @@ def chromatic_polynomial(G, return_tree_basis=False, algorithm='C', cache=None):
             mpz_mul(coeff, coeff, m)
     coeffs_ZZ = []
     cdef Integer c_ZZ
-    for i from 0 <= i <= nverts:
+    for i in range(nverts + 1):
         c_ZZ = Integer(0)
         mpz_set(c_ZZ.value, coeffs[i])
         coeffs_ZZ.append(c_ZZ)
     f = R(coeffs_ZZ)
 
-    for i from 0 <= i <= nverts:
+    for i in range(nverts + 1):
         mpz_clear(tot[i])
         mpz_clear(coeffs[i])
 

src/sage/graphs/matchpoly.pyx

@@ -216,14 +216,14 @@ def matching_polynomial(G, complement=True, name=None):
 
     cdef int i, j, d
     cdef fmpz_poly_t pol
-    cdef nverts = G.num_verts()
+    cdef int nverts = G.num_verts()
 
     # Using Godsil's duality theorem when the graph is dense
 
     if complement and G.density() > 0.5:  # this cutoff could probably be tuned
         f_comp = matching_polynomial(G.complement()).list()
         f = x.parent().zero()
-        for i from 0 <= i <= nverts / 2:  # implicit floor
+        for i in range(nverts // 2 + 1):
             j = nverts - 2 * i
             f += complete_poly(j) * f_comp[j] * (-1)**i
         return f