Switch to use uv.lock instead of test-requirements.txt

[CoolCat467]

In this pull request, we switch the CI system to use uv.lock instead of test-requirements.txt.

[A5rocks]

I don't like this change (because I install the dependencies using pip install -r test-requirements.txt!), so I'm a -1 here.

My opinion might shift if there's ever a standardized lockfile format though.

[jakkdl]

I don't like this change (because I install the dependencies using pip install -r test-requirements.txt!), so I'm a -1 here.

My opinion might shift if there's ever a standardized lockfile format though.

presumably one could use uv sync tests for the same effect? I think that might be faster and stuff. But afaict the current setup works fairly well so I don't have strong opinions either way - maybe time to revisit #2790 and see which, if any, of the pain points brought up are still a thing.

[A5rocks]

Actually, there's a standardized lockfile now. Pip's support for it is in beta, but I figure we can switch to it whenever. In particular, it looks like we can swap pip compile requirements.in --universal -o requirements.txt to uv pip compile requirements.in -o pylock.toml (I don't know if we can do test-requirements.in and docs-requirements.in in the same file...)