Replace Tox test suite with an alternative

[Archmonger]

Description

Tox is a bit of a nightmare to work with it's fiddly configuration files, so we should switch to that when possible.

[stumpylog]

You might consider something like hatch as well. All configured via pyproject.toml, it can use matrices to provide a full replacement of tox, while also handling building sdist and wheels. Just a thought.

Thanks for the fork, I'm looking into using it over whitenoise.