Tox v4 behaves significantly differently than v3, and some of the
more complex things we do with tox would need an overhaul to
continue to use it. Meanwhile, nox is much simpler and more
flexible, so let's try using it.
This adds a noxfile which should be equivalent to our tox.ini file.
We still need to update the docs build (which involves changes to
base jobs) before we can completely remove tox.