Add tox for easy testing of multiple Python versions

[hugovk]

For example:

# Test with one Python version (e=environment)
tox -e py312

# Test with all installed versions
tox

# Test with all installed versions, in parallel
tox -p auto

Also use tox on the CI, test with coverage, and upload coverage to Codecov.

Coverage is an impressive 91.29%!

Also adds PyPy3.10 to the test matrix.

[ezio-melotti]

Does this PR replace #68?

[hugovk]

It replaces some of it, but that also includes some linting stuff. I was also planning on adding some pre-commit based linting after this is merged.

[JulienPalard]

I have a very bad experience with pytest-cov running in parallel (inside tox -p all), to the point I took the habit to just use coverage run -m pytest (which is not much more to type).

See: pytest-dev/pytest-cov#416

Is your experience different? Maybe it has been somehow fixed by now?

[hugovk]

Hmm, I often use tox -p auto and haven't noticed that.

I added this to the top of sphinxlint.py:

import sys

if sys.version_info >= (3, 10):
    a = 1
else:
    a = 2

Ran tox -p all and it showed as covered:

Maybe it's fixed? Or I'm not running into it for some reason?

[JulienPalard]

Maybe it's fixed? Or I'm not running into it for some reason?

Maybe.

If we spot sqlite/coverage related errors, we know how to fix it (but just not using pytest-cov).

Let's try your way.

[hugovk]

Sounds good.