fix: Python 3.7 support

[henryiii]

This restores Python 3.7 support, and adds testing for it, and adds static checking to keep this bug from popping up again. I already had _compat.* work started, and ran into a bug in Ruff that kept .typing from being used, so I went ahead and used that to work around the Ruff bug. I'm using a _compat.<stdlib> form for most of my other projects, and am happy with it.

• ci: add Python 3.7
• fix: restore Python 3.7 support

Fixes #1478. See also astral-sh/ruff#4013.

TODO:

• I need to restore non-backported typing to cibuildwheel.typing.
• Add tomllib to _compat

[mayeut]

Python 3.7 support issue should have been caught by Travis CI. It exits with a successful status even when there are errors.

[joerick]

I'll merge this as I'd like to get a bug fix release out, the _compat thing looks good to me. I agree with @mayeut though, we should have had CI coverage for 3.7 on Travis, so we should figure out what happened there.

[joerick]

Thanks for fixing so quickly @henryiii. I've released a new version with the fix. Created #1480 for the CI check, I don't think we'll need this extra Github Actions check long-term.