refactor: clean up packaging

[henryiii]

Proposed updates to packaging. I'm trying Hatch here; I've used it in a few places but not on the cluster yet, but I think it might work well on the cluster. If we pull any conda specifics, we can try pixi. Open to suggestions if others prefer something else. I can move the environment config from pyproject.toml to hatch.toml if people like that better. I generally like fewer files, but am 50:50 on that one. Hatch (and nox and pixi) are 100x faster than other alternatives due to uv support.

I deleted all the stuff that was irrelevant or well out of date, like the coverage stuff (there are no tests), the tox stuff (3.6-3.8 for a 3.10+ project), etc.

I moved the backend from setuptools to hatchling and deleted all the setuptools-specific files.

I setup a spot for tests outside the source dir instead of inside it.

[henryiii]

I don't have access to merge, if you mean me. :)

[henryiii]

Building the docs on readthdocs went from 2 minutes to about 21 seconds. :)

(111 seconds to 3 seconds for the install step)