python@3.10: remove include_dirs, library_dirs from distutils.cfg

[mkoeppe]

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Homebrew's python3 installs distutils.cfg, which injects include and library paths into the build of every user package. The injected directories appear at the beginning of the paths and hence take precedence over paths configured by the packages' build systems. (We encountered this in the SageMath project in https://trac.sagemath.org/ticket/31335; our workaround is to use SETUPTOOLS_USE_DISTUTILS=local, thus disabling homebrew's copy of distutils.)
In this PR, we remove one item from these paths, with the eventual goal of removing distutils.cfg altogether. distutils.cfg is an outdated and problematic mechanism, which is moreover set to be ineffective for packages that use setuptools when setuptools (again) makes SETUPTOOLS_USE_DISTUTILS=local the standard behavior (pypa/setuptools#2896). See also #76621.

[Bo98]

I'd rather the include/library directories were all dealt with at the same time rather than removing a random one. I definitely agree they should go, even if I'm a little cautious at what we're potentially breaking here.

(I'm aware of the future setuptools issue in regards to prefix but I am already working on a solution for that)

The injected directories appear at the beginning of the paths and hence take precedence over paths configured by the packages' build systems.

More of a side note this, but this is pretty much specific to your use of the compiler-level CPATH. Unless something changed recently, CFLAGS and the use of Extension's include_dirs in setup.py don't behave this way.

[mkoeppe]

I'd rather the include/library directories were all dealt with at the same time rather than removing a random one. I definitely agree they should go, even if I'm a little cautious at what we're potentially breaking here.

OK, I have made this change

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[mkoeppe]

ping?

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[Bo98]

Apologies for the late response. This has been merged as a part of 3.10.2, but I forgot to mention this issue in the commit. We decided it was best to remove distutils.cfg entirely all at the same time rather than selectively remove parts over time. This took a little bit of time to get right since we needed to modify stuff like sysconfig to ensure we retain the existing install prefix behaviour, as well as work with upstream to make some changes to setuptools. It's all merged now however and we're monitoring if anything has broken so let us know if it makes things better or worse for you. I will post an update to #76621 when I'm comfortable that everything is working fine.

I'm not entirely sure yet what we'll backport to Python 3.9 and earlier. We could backport this bit if needed, but there'll still be some form of distutils.cfg that's around for those versions.

[mkoeppe]

Thanks for the update!