Add CI based on 'maturin generate-ci'

[djmitche]

This adds a simple CI workflow, so we can check that tests are still passing, etc.

[djmitche]

@illya-laifu any idea what's up here? I asked a Q&A in PyO3/maturin#2412.

[illya-laifu]

According to the linked Github issue in the error:

FYI, if you ended up here because your Github Action reported your Python version (e.g. 3.8) was not compatible with a GIL-free build of Python, check your configuration of the requires-python of your pyproject.toml file and make sure it's 3.9 or above.

So all that has to be done, is this:
python = ">=3.8"

has to be bumped to 3.9 or higher.

[illya-laifu]

Glad the solution was just this simple 👌