Switch to using uv for dev and testing

[enocom]

https://docs.astral.sh/uv/

We can move our requirements.txt and requirements-test.text into pyproject.toml test sections and delete those files for a better user experience.

We can also delete nox. We'll also need to commit our ux.lock file.

[enocom]

Note this won't affect any of our release infrastructure. We'll continue using pyproject.toml for that.