Use stdlib for setuptools on Cygwin

[radarhere]

Alternative to #6759

pyroma 4.1 was released yesterday. It now requires setuptools >= 61. This has started Cygwin failing in main - https://github.com/python-pillow/Pillow/actions/runs/3544062653

This PR uses SETUPTOOLS_USE_DISTUTILS=stdlib, as suggested previously.

[hugovk]

Thanks!

The plan is that distlib will be removed from the stdlib in Python 3.12:

• https://peps.python.org/pep-0632/
• https://setuptools.pypa.io/en/latest/deprecated/distutils-legacy.html

Python 3.12 final is due out in October 2023, with an alpha already out:

• https://peps.python.org/pep-0693/

We should start testing 3.12 at some point, I think before the beta period (May 2023) would make sense, because there shouldn't be any feature changes during the beta, and if there's big problems with setuptools/distlib we may be able to give useful feedback before it's too late. (Having said that, for Cygwin we've not been testing pre-releases.)

So we may need to revisit this. But we may see more churn in setuptools before then, so I think this is fine for the time being. 👍