Use tox to manage the test suite

[kurtmckee]

This PR introduces tox to manage the test suite. This includes the following changes:

• The test suite is now run on Python 3.8 through Python 3.11 automatically.
• Coverage reports include runs across all Python versions, not just Python 3.8.
• The coverage report include coverage of the tests/ directory.
• The documentation is built as a part of the test suite.
• pandas is now recognized as a test dependency (previously the test suite would fail when pandas was not manually installed).
• CI now runs the test suite across all Python versions, including building the docs.

To run the test suite locally, simply install tox (pip install tox) and run tox. To run the tests in parallel, run tox parallel or tox p.

[kurtmckee]

Rebased on main to resolve merge conflict

[kurtmckee]

It appears the Sonar test is failing. This is very likely a security consideration that prevents external repositories from accessing this repository's secrets.

I looked through past runs of CI and found that they appear to succeed when they're running against main and to fail when they're running against PR branches.

I'm not yet familiar with the security considerations for token access, but I don't think that the CI failure is happening because of a change introduced in this PR.

[phschiele]

@kurtmckee yes, sonar is failing because of the missing secret.

I haven't used tox before, so I'm reading up a bit before merging. I like the idea of moving logic from the CI bash scripts to the tox configuration.

[kurtmckee]

Sounds good. Let me know if you have any questions or see any changes that are needed.

BTW, I saw you're giving some talks at SciPy 2023! I'm looking forward to it!

[phschiele]

Thanks, this is indeed a great functionality. See you at the talks!