Bring package into the future of 2019

[Fuyukai]

I saw a book entitled "Die setup.py" and I thought "My feelings exactly". Turns out the book was in German.

(Paraphrased).

This brings h11 into the glorious future year of 2019, removing setup.py and replacing it with a working pyproject.toml. It also does the following:

• Sets up Ruff as a linter with all rules enabled except for a handful
• Sets up Pyrefly as the type checker and fixes (nearly) all type errors picked up
• Updates the code base to Python 3.10
• Drops support for the geriatic Python 3.8 and 3.9.

Tox setup is still in its own file because, well, I don't get Tox. This also updates the classifier from Alpha to Beta (although it's not like anyone actually reads those).

The test suite passes on all versions, and httpcore's test suite passes when this PR is installed manually too.