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: https://github.com/sagemath/sage/pull/37290
Reported by: Gonzalo Tornaría
Reviewer(s): Gonzalo Tornaría, Matthias Köppe
Committer
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