MNT: migrate requirement files to PEP 735 dependency groups

[neutrinoceros]

No description provided.

[neutrinoceros]

This mostly works but wheel building is now broken (checked by manually triggering the workflow).
What seems to be happening is that:

• uv sync supports --(only-)group but doesn't allow specifying a custom location for the virtual env¹, so it's basically useless in the context of cibuildwheel (which creates a virtual env in an arbitrary location, intentionally outside the project dir)
• uv pip install doesn't support --(only-)group options. It's not clear to me at the moment if this is a temporary or permanent limitation.

I've opened pypa/cibuildwheel#2057 to ask for native support of dependency groups in cibuildwheel.

Footnotes

1. I was wrong, this part is actually configurable via an env variable, but so far I can't make this work either ↩

[neutrinoceros]

This should be ready now thanks to the release of cibuildwheel 2.22

watch https://github.com/neutrinoceros/gpgi/actions/runs/11985331508