Switch to uv

[ghickman]

⚠️ Do not merge until #239 is in, I don't to update the readme twice! ⚠️

This switches the project from using uv's pip and pip-compile interfaces to the full pyproject-configured usage.

Some benefits:

• All configuration is now in pyproject.toml, including the dependency specs from requirements.*.in
• uv run lets us treat virtualenvs and pythons as an implementation detail of the project, uv will make sure we have everything set up correctly
• The required python version is now defined in a standards-compliant fashion in the project block of pyproject.toml.

Note: dependency caching in CI is on by default on GitHub-hosted runners with setup-uv, which is why there is no caching configuration for that action.