Use uv to manage the project and handle CI setup

[masklinn]

• ua-parser-builtins means this has already become a hacked-together workspace, uv has built-in support for workspaces
• might have better support for development tools via uv tool?
• first-class support for dev dependencies and extras (--extra <dep>, --all-extra, --dev)
• can take a pyproject.toml as requirements, which allows installing just the dependencies of a package, something which... might be useful to cleanup the CI setup some?
• tox-uv
• astral provides builtin github actions, which could help clean up the repo's (certainly can't make them worse)
• both pypy (Support PyPy astral-sh/uv#2096) and graal (Add GraalPy support astral-sh/uv#5141) seem supported
• TODO: check if working in the workspace (heh) makes the workspace members implicitly depend on one another or if they go and fetch from pypi, the former would be very convenient for some changes which cross releases, or experiments.