add tox config

[jakkdl]

I wrote a basic tox config when writing #281 as I was fighting with various version differences and kept failing the CI, and figured other people might find it useful as well.

[cooperlees]

Cool, as much as I dislike too, I like it's helpfulness + uiquotious nature in the python OSS ecosystem so happy to use it here.

If you have time - Want to change our GitHub action to use it to so we know it's working? If not, I can eventually get to it.

[jakkdl]

Done! Looked around a bit and https://github.com/ymyzk/tox-gh-actions looked like the best way to do it. Also updated documentation and added tox to [dev], although it's somewhat redundant to have both tox and coverage+hypo*

Looks like wheel isn't needed to be installed in the CI anymore, though I admit I only found that out through brute force testing so maybe it's needed for some other reason.

[cooperlees]

Thanks. Let's move to tox life.