add initial tox.ini

[jakkdl]

I started out on a tox.ini because it's handy to have; it's not complete enough to close #3170, but it should be sufficient for #2699 once the CI is updated to use it.

• there's a bunch missing to completely get rid of ci.sh, I almost suspect we might need nox for that not to be a nightmare.
• I probably prefer having this in pyproject.toml, which tox now supports natively, but being so used to tox.ini I didn't realize this until most of the file was written.
• I haven't switched over anything in ci.yml to actually use this. We'd probably want https://github.com/tox-dev/tox-gh or https://github.com/ymyzk/tox-gh-actions to do that properly

everybody's very welcome to push commits to this PR, but I don't think we should block merging it until the tox/nox config is perfect (leaving that for followup PRs) Cause I Wanna Use It ™️