support for PEP 735 (`dependency-groups`)

[cpcloud]

Similar to poetry's dependency groups, there's now an official way to specify these in a canonical pyproject.toml and it's supported in uv.

[adisbladis]

I started adding PEP-735 support to pyproject.nix the other day: https://github.com/nix-community/pyproject.nix/tree/pep735.
It's not done yet (still needs an interface in the project abstraction).