Run tests with Nox instead of tox #1734

We currently use tox to automate our tests. It is a really good tool, with the tradeoff that the configuration file, tox.ini, is in a rather esoteric format which makes it hard to update & edit tests. (At the very least, I personally have trouble understanding it.) Another limitation is that it's hard to dynamically generate test environments.

An alternative is nox, which is similar to tox, except that the test configuration file is written in Python. We're a lot more familiar with Python, so that will make the tests easier to maintain. Plus, we can more easily dynamically generate tests.

One motivation for switching to nox would be that we would be able to get the minimum requirements from our requirements files, rather than having to maintain requirements in one more place (currently in our tox.ini file). This would help us with #1276.

A while back I set up a noxfile.py for xrtpy, and was thinking it would be worthwhile to set up a prototype for PlasmaPy as well.

If we do switch to nox, we would also need to update our testing guide.