gh-37290: sagemath-standard: don't require Cython to create sdist

The implementation of sage.misc.package_dir.read_distributions uses the
function `Cython.Utils.open_source_file()` to open a file instead of
`io.open()`. This function was introduced in #29701 (b582789).

The only difference between `open_source_file()` and `io.open()` is that
the former will open the file as binary to check the encoding. But all
our files should be utf-8 so we don't need this trick.

In case I’m mistaken and there is a reason this trick is really
necessary (@mkoeppe?) it seems better to include a version of
open_source_file(), for two main reasons: (a) we don’t pull the whole of
Cython just for a trivial function; (b) changes in Cython don’t affect
sagemath (this is not a documented function of Cython, afaict)

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URL: #37290
Reported by: Gonzalo Tornaría
Reviewer(s): Gonzalo Tornaría, Matthias Köppe

Author: Release Manager

build/pkgs/configure/checksums.ini

@@ -1,4 +1,4 @@
 tarball=configure-VERSION.tar.gz
-sha1=84c8bfc8a84399d50eac5f0e75672da0e36216ae
-md5=1171b0a7cd69002c426fb7b48e9026e4
-cksum=2998629640
+sha1=6517328415e972b571db0ef97c780ae45bb90745
+md5=671ce9d8afcf9ad5faa6a918d7ab2829
+cksum=2336342933

build/pkgs/configure/package-version.txt

@@ -1 +1 @@
-128d437fcc7f3917b314662fdee34aeaab968b67
+8dd853eddb146b36c5ce10362ac9b4ca5faf622b

src/sage/misc/package_dir.py

@@ -118,8 +118,7 @@ def read_distribution(src_file):
         sage: read_distribution(os.path.join(SAGE_SRC, 'sage', 'graphs', 'graph_decompositions', 'modular_decomposition.py'))
         ''
     """
-    from Cython.Utils import open_source_file
-    with open_source_file(src_file, error_handling='ignore') as fh:
+    with open(src_file, encoding='utf-8', errors='ignore') as fh:
         for line in fh:
             # Adapted from Cython's Build/Dependencies.py
             line = line.lstrip()