chore: move site-requirements to a PEP 735 dependency group

[woodruffw]

We now have an in-repo pyproject.toml, thanks to the maturin builds. As such, we can remove the site-requirements.txt file and replace it with a PEP 735 dep group, which is already well-supported by uv.

See: https://peps.python.org/pep-0735/

[woodruffw]

Took a look at this, and this might not be possible yet: it's possible to do uvx --with-requirements but not uvx --with-group or similar.

OTOH, I might be taking the wrong approach here -- uv tool install or similar might be the intended way to do this.