Switch to `pylock.toml` from our `test-requirements.txt` lockfile

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...)

Originally posted by @A5rocks in https://github.com/python-trio/trio/issues/3188#issuecomment-4413943729

Start by reviewing the current requirements.in, test-requirements.in, and docs-requirements.in inputs and the pip-compile command that produces requirements.txt. Check uv pip compile's pylock.toml support and how multiple input files should be represented; done means the existing lockfile process is replaced with a working pylock.toml-based process.