gh-40063: avoid using bytes_to_str in combinat
    
using instead the builtin decode method of bytes

### 📝 Checklist

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
    
URL: #40063
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert

Author: Release Manager

src/sage/combinat/designs/covering_design.py

@@ -54,7 +54,6 @@
 from sage.arith.misc import binomial
 from sage.combinat.combination import Combinations
 from sage.combinat.designs.incidence_structures import IncidenceStructure
-from sage.cpython.string import bytes_to_str
 
 
 def schonheim(v, k, t):
@@ -520,11 +519,8 @@ def best_known_covering_design_www(v, k, t, verbose=False):
     if verbose:
         print("Looking up the bounds at %s" % url)
 
-    f = urlopen(url, context=default_context())
-    try:
-        s = bytes_to_str(f.read())
-    finally:
-        f.close()
+    with urlopen(url, context=default_context()) as f:
+        s = f.read().decode()
 
     if 'covering not in database' in s:  # not found
         str = "no (%d, %d, %d) covering design in database\n" % (v, k, t)

src/sage/combinat/matrices/hadamard_matrix.py

@@ -90,7 +90,6 @@
                                                      skew_supplementary_difference_set,
                                                      complementary_difference_sets)
 from sage.combinat.t_sequences import T_sequences_smallcases
-from sage.cpython.string import bytes_to_str
 from sage.rings.integer_ring import ZZ
 from sage.rings.polynomial.polynomial_ring_constructor import PolynomialRing
 from sage.matrix.constructor import (block_matrix,
@@ -1964,7 +1963,7 @@ def hadamard_matrix_www(url_file, comments=False):
     rws = []
     url = "http://neilsloane.com/hadamard/" + url_file
     with urlopen(url) as f:
-        s = [bytes_to_str(line) for line in f.readlines()]
+        s = [line.decode() for line in f.readlines()]
     for i in range(n):
         line = s[i]
         rws.append([1 if line[j] == "+" else -1 for j in range(n)])