Add support for Python 3.10

[hugovk]

Tracking issue for getting the pieces into place to support Python 3.10 in Pillow 8.4.0, scheduled for 2021-10-15.

• Wait for 3.10.0, expected Monday, 2021-10-04
• Wait for 3.10.0 on GHA: Support 3.10 actions/setup-python#249
• Wait for 3.10.0 on AppVeyor: Python 3.10 support appveyor/ci#3741
• (pillow-wheels#243) Wait for 3.10.0 on Travis CI (https://github.com/python-pillow/pillow-wheels): https://travis-ci.community/t/add-python-3-10/12220
• (#5748) Replace 3.10-dev with 3.10 on GHA
• (#5805) Replace 3.9 with 3.10 on AppVeyor
• (#5570) Add Trove classifier, update setup.py version check, add to tox.ini, update Python support table
• (#5713) Update CI targets table
• (python-pillow/pillow-wheels#215 python-pillow/pillow-wheels#217) Update https://github.com/python-pillow/pillow-wheels

(3.9 issue for comparison: #4953)

[wiredfool]

I've seen this mentioned elsewhere and it makes sense -- Shipping a py 3.10 compatible/tagged wheel before the actual release can help with other libraries sorting their compatibility. It could reduce the support load here as well, as we wouldn't get the confusion from people attempting to compile for 3.10 in the window between it coming out and us (well, you) getting the new release out. This assumes that we can run the RC in the testing setup, and I'm not 100% clear that that's actually the case.

In the unlikely event that the 3.10 RC differs from 3.10 final in a significant manner, we can roll a point release.

[hugovk]

We have an 11 day buffer between 3.10.0 final release and Pillow release, and I was going to suggest doing the Pillow release as soon as we're actually ready, before the end of that buffer.

But yes, sounds good to put out RC wheels first. We've already been testing this repo against 3.10 on GHA.

What do we need?

Windows

https://www.lfd.uci.edu/~gohlke/pythonlibs/#pillow already has 3.10 wheels for the newest Pillow 8.3.1, so we could upload those right away (or first check/ask for a build against RC1).

macOS and Linux

Update https://github.com/python-pillow/pillow-wheels for 3.10, first we need https://github.com/matthew-brett/multibuild to add support?

multibuild depends on https://github.com/pypa/manylinux which already supports 3.10.0 RC1: pypa/manylinux#1159.

[radarhere]

pillow-wheels now has 3.10.0rc1 - python-pillow/pillow-wheels#215 and python-pillow/pillow-wheels#217

[hugovk]

Great, thanks for those!

What's the easiest way for us to proceed? Do we need to make some code changes before creating wheels, and how best to trigger the wheel build process?

Perhaps a point release (8.3.2) with #5570 (or a subset)? And anything else?

[radarhere]

The wheels have actually already been generated. From https://github.com/python-pillow/pillow-wheels/actions/runs/1170567136 I can download the artifacts. Here are just the Python 3.10 wheels - Pillow-8.3.1-cp310-cp310.zip.

#5570 is needed to get rid of the exit warning on Windows though, so 8.3.2 probably sounds like the best way to go.

If we're looking for 8.3.0 problems to fix, then I would suggest #5588, since that problem was introduced in #5514, and #5599 since that adds documentation changes missing from #5552.

[hugovk]

Yep, fine by me to include those regression fixes too.