Add Python 3.10 testing.

[QuLogic]

PR Summary

Currently, pillow (python-pillow/Pillow#5569) and kiwisolver (nucleic/kiwi#103) do not have wheels, which slows down the build a bit, but it should pass anyway.

PR Checklist

• Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• [n/a] New features are documented, with examples if plot related.
• [n/a] Documentation is sphinx and numpydoc compliant (the docs should build without error).
• [n/a] Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• [n/a] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• [n/a] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).

[QuLogic]

Oh, looks like we also need tornado, pandas, pikepdf, pandocfilters, and pyrsistent wheels, but pikepdf is the one tripping us up due to its C libraries.

[QuLogic]

I opened pikepdf/pikepdf#245 and disabled extra test dependencies, but it might be a while since Pillow doesn't have wheels either.

[QuLogic]

So the failures are now:

• Qt5 with PySide2: TypeError: 'PySide2.QtCore.Qt.ToolBarArea' object cannot be interpreted as an integer; @tacaswell was worried we broken PySide2 when we added Qt6 support, but reverting the _enum still fails. I think this is a PySide2 incompatibility and might not be fixed by them either.
• test_fork, test_savefig_backend, test_load_from_url, test_use_url: ImportWarning: DynamicImporter.find_spec() not found; falling back to find_module()` -> https://gitlab.gnome.org/GNOME/pygobject/-/issues/473
• test_tinypages: ImportError: cannot import name 'Union' from 'types' -> Sphinx doesn't support python 3.10 sphinx-doc/sphinx#9562

[jkseppan]

Is it possible to make the 3.10 test optional, so it doesn't cause a red "Some checks were not successful" message on Github?

[dopplershift]

GitHub actions doesn't have a notion of "allowed failures"--every PR workflow shows up with its status here. (This has been frustrating) The options are:

• Manually override and mark as passing (seems silly)
• Make the Python 3.10/pre-release build something that runs periodically on a cron (e.g. nightly). For this one it's possible to have the workflow open an issue if it fails.

[tacaswell]

We could forgive the ImportWarning's in the configuration (like we are doing with deprecation warnings from fontTools right now (and fontTools fixed that upstream yesterday))?

I'll look at the pyside2 issue and the sphinx issue as already fixed on their default branch.

[QuLogic]

We could forgive the ImportWarning's in the configuration (like we are doing with deprecation warnings from fontTools right now (and fontTools fixed that upstream yesterday))?

Oh yea, we could do that.

I'll look at the pyside2 issue and the sphinx issue as already fixed on their default branch.

I have a work around for PySide2 mostly complete.

[tacaswell]

I have a work around for PySide2 mostly complete.

Great, I was just about to start on this and now will not :)

[QuLogic]

The change to enums is enough to work around PySide2 in tests for me, but if I actually run an example, it crashes in a NumPy copy during a paint operation. It doesn't happen in TkAgg or PyQt5, so I'm not sure what I can do about it.

[QuLogic]

Note the previous typo in the refactor of Qt5/Qt6 code together.

[QuLogic]

Ditto.