Use tox

[micahellison]

We're thinking about using tox for configuration and possibly simplifying our GitHub Actions pipelines. Switching to it could fix a few pain points:

• quickly testing locally on a variety of Python versions
• being able to test both locally and on GitHub Actions with mostly the same code
• possibly being able to sidestep some environment/syntax issues we've had between *nix/bash and Windows/PowerShell on GH Actions
• replacing the makefile, allowing Windows users to quickly use build automation without having to deal with Make for Windows (which hasn't been updated since 2006)

Poetry supports tox and has some notes on using it in its docs. We just merged in #1359 which resolves the pre-req it mentions by using poetry-core instead of poetry.

Feel free to chime in with pros/cons or any complications that might arise.