Python >=3.9 compat, update packaging

[sneakers-the-rat]

@phildong got this started for ya last night, basically i just updated the packaging to modern pyproject.toml style, set a new minimum, ran ruff's pyupgrade formatter, and then started running the tests and whacking away at whatever bugs started to happen. mostly just simple API changes in dependencies. shouldn't take too much longer to get it working. bonus: this should also resolve many of the installation issues i'm seeing as well as make it possible to put this up on pypi.

opening as a draft with allow edits by maintainers, you should be able to push directly to this branch like

git remote add sneakers-the-rat https://github.com/sneakers-the-rat/minian
git fetch sneakers-the-rat
git switch -c py39 sneakers-the-rat/py39
# do stuff... commit... push as normal

[sneakers-the-rat]

wasn't sure why version was defined in 4 different places, so reduced it to only two (not sure what that committizen tool does)