gh-40072: avoid using bytes_to_str in matrix/ (pyx files)
    
<using instead the builtin `encode` or `decode` methods.

### 📝 Checklist

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
    
URL: #40072
Reported by: Frédéric Chapoton
Reviewer(s): David Coudert

Author: Release Manager

src/sage/matrix/matrix_gfpn_dense.pyx

@@ -44,7 +44,6 @@ cimport cython
 #
 ####################
 
-from sage.cpython.string cimport str_to_bytes
 from sage.cpython.string import FS_ENCODING
 from sage.rings.integer import Integer
 from sage.rings.finite_rings.finite_field_constructor import GF
@@ -467,8 +466,8 @@ cdef class Matrix_gfpn_dense(Matrix_dense):
             raise ValueError("cannot construct meataxe matrix from empty filename")
 
         if type(filename) is not bytes:
-            filename = str_to_bytes(filename, FS_ENCODING,
-                                    'surrogateescape')
+            filename = filename.encode(FS_ENCODING, 'surrogateescape')
+
         sig_on()
         try:
             mat = MatLoad(filename)
@@ -1906,7 +1905,9 @@ def mtx_unpickle(f, int nr, int nc, data, bint m):
     # in Python-3, Sage will receive a str in `latin1` encoding. Therefore,
     # in the following line, we use a helper function that would return bytes,
     # regardless whether the input is bytes or str.
-    cdef bytes Data = str_to_bytes(data, encoding='latin1')
+    if isinstance(data, str):
+        data = data.encode('latin1')
+    cdef bytes Data = data
     if isinstance(f, int):
         # This is for old pickles created with the group cohomology spkg
         MS = MatrixSpace(GF(f, 'z'), nr, nc, implementation=Matrix_gfpn_dense)

src/sage/matrix/matrix_integer_dense.pyx

@@ -64,7 +64,7 @@ TESTS::
 from libc.stdint cimport int64_t
 from libc.string cimport strcpy, strlen
 
-from sage.cpython.string cimport char_to_str, str_to_bytes
+from sage.cpython.string cimport char_to_str
 from sage.ext.stdsage cimport PY_NEW
 from cysignals.signals cimport sig_check, sig_on, sig_str, sig_off
 from cysignals.memory cimport sig_malloc, sig_free, check_allocarray
@@ -497,7 +497,7 @@ cdef class Matrix_integer_dense(Matrix_dense):
             sage: matrix(ZZ,1,3,[1,193,15])._pickle() == (b'1 61 f', 0)   # indirect doctest
             True
         """
-        return str_to_bytes(self._export_as_string(32), 'ascii')
+        return self._export_as_string(32).encode('ascii')
 
     cpdef _export_as_string(self, int base=10):
         """
@@ -582,18 +582,18 @@ cdef class Matrix_integer_dense(Matrix_dense):
                 self._unpickle_matrix_2x2_version0(data)
             else:
                 raise RuntimeError("invalid pickle data")
+
         else:
-            raise RuntimeError("unknown matrix version (=%s)" % version)
+            raise RuntimeError(f"unknown matrix version (={version})")
 
     cdef _unpickle_version0(self, data):
-        cdef Py_ssize_t i, j, n, k
+        cdef Py_ssize_t i, j, k
         data = data.split()
-        n = self._nrows * self._ncols
-        if len(data) != n:
+        if len(data) != self._nrows * self._ncols:
             raise RuntimeError("invalid pickle data")
         k = 0
-        for i from 0 <= i < self._nrows:
-            for j from 0 <= j < self._ncols:
+        for i in range(self._nrows):
+            for j in range(self._ncols):
                 s = data[k]
                 k += 1
                 if fmpz_set_str(fmpz_mat_entry(self._matrix, i, j), s, 32):

src/sage/matrix/matrix_mod2_dense.pyx

@@ -118,7 +118,7 @@ from sage.misc.verbose import verbose, get_verbose
 VectorSpace = None
 from sage.modules.vector_mod2_dense cimport Vector_mod2_dense
 from sage.structure.richcmp cimport rich_to_bool
-from sage.cpython.string cimport bytes_to_str, char_to_str, str_to_bytes
+from sage.cpython.string cimport char_to_str
 from sage.cpython.string import FS_ENCODING
 
 cdef extern from "gd.h":
@@ -480,17 +480,17 @@ cdef class Matrix_mod2_dense(matrix_dense.Matrix_dense):   # dense or sparse
                 div_s[2*i] = c'+'
                 last_i = i
 
-        for i from 0 <= i < self._nrows:
+        for i in range(self._nrows):
             row_s = row = b'[' + empty_row + b']'
-            for j from 0 <= j < self._ncols:
-                row_s[1+2*j] = c'0' + mzd_read_bit(self._entries,i,j)
+            for j in range(self._ncols):
+                row_s[1+2*j] = c'0' + mzd_read_bit(self._entries, i, j)
             s.append(row)
 
         if self._subdivisions is not None:
             for i in reversed(row_div):
                 s.insert(i, row_divider)
 
-        return bytes_to_str(b"\n".join(s))
+        return (b"\n".join(s)).decode()
 
     def row(self, Py_ssize_t i, from_list=False):
         """
@@ -2572,7 +2572,7 @@ def from_png(filename):
     fn.close()
 
     if type(filename) is not bytes:
-        filename = str_to_bytes(filename, FS_ENCODING, 'surrogateescape')
+        filename = filename.encode(FS_ENCODING, 'surrogateescape')
 
     cdef FILE *f = fopen(filename, "rb")
     sig_on()
@@ -2619,7 +2619,7 @@ def to_png(Matrix_mod2_dense A, filename):
     fn.close()
 
     if type(filename) is not bytes:
-        filename = str_to_bytes(filename, FS_ENCODING, 'surrogateescape')
+        filename = filename.encode(FS_ENCODING, 'surrogateescape')
 
     cdef gdImagePtr im = gdImageCreate(c, r)
     cdef FILE * out = fopen(filename, "wb")

src/sage/matrix/matrix_rational_dense.pyx

@@ -73,7 +73,7 @@ Test hashing::
 from libc.string cimport strcpy, strlen
 
 from sage.categories.rings import Rings
-from sage.cpython.string cimport char_to_str, str_to_bytes
+from sage.cpython.string cimport char_to_str
 
 from sage.modules.vector_rational_dense cimport Vector_rational_dense
 from sage.ext.stdsage cimport PY_NEW
@@ -374,13 +374,13 @@ cdef class Matrix_rational_dense(Matrix_dense):
                 s = data[k]
                 k += 1
                 if '/' in s:
-                    num, den = [str_to_bytes(n) for n in s.split('/')]
+                    num, den = (n.encode() for n in s.split('/'))
                     if fmpz_set_str(fmpq_mat_entry_num(self._matrix, i, j), num, 32) or \
                        fmpz_set_str(fmpq_mat_entry_den(self._matrix, i, j), den, 32):
                         raise RuntimeError("invalid pickle data")
                 else:
-                    s = str_to_bytes(s)
-                    if fmpz_set_str(fmpq_mat_entry_num(self._matrix, i, j), s, 32):
+                    num = s.encode()
+                    if fmpz_set_str(fmpq_mat_entry_num(self._matrix, i, j), num, 32):
                         raise RuntimeError("invalid pickle data")
                     fmpz_one(fmpq_mat_entry_den(self._matrix, i, j))