Use uv pip in CI?

uv pip can significantly speed up CI runs, especially in cases where the dependency tree is large. This Uncle Val PR is a proof of concept, and it reduces the total* test CI running time from ~5 minutes to <3 minutes.

* - Total here means total time for a single workflow, e.g., testing with Python 3.13 only. I don't compare dependency installation step times alone because it is not fair: pip does byte-compile the code, while uv pip doesn't do it by default.