Pin to Python 3.9.16-1

[radarhere]

Our Cygwin jobs have been failing for several days - https://github.com/python-pillow/Pillow/actions/workflows/test-cygwin.yml

In looking at a discussion over at NumPy about this, there was a suggestion to use the egor-tension/setup-cygwin action and pin to Python 3.9.16-1 - numpy/numpy#25708 (comment)

[hugovk]

Sounds like a good idea. Has this been reported upstream to Cygwin?

[radarhere]

I've just posted in the Cygwin mailing list - https://cygwin.com/pipermail/cygwin/2024-January/255273.html

[hugovk]

So before, both 3.8 and 3.9 jobs were timing out: https://github.com/python-pillow/Pillow/actions/runs/7705171752

Now, we've switched to egor-tensin/setup-cygwin but does python39=3.9.16-1 mean we install that for both 3.8 and 3.9 jobs? But we're still testing with either 3.8 or 3.9 for the relevant job.

https://github.com/python-pillow/Pillow/actions/runs/7708554081

[radarhere]

python39 is already being installed in the 3.8 job, as some sort of dependency - https://github.com/python-pillow/Pillow/actions/runs/7705171752/job/20998682687#step:4:362. It's just explicit rather than implicit now so that the version can be pinned.

[radarhere]

This was reverted in #7880