Switch to using uv for Python dependencies

[robhudson]

This is a draft PR to explore the idea and discuss.

The workflow then becomes...

1. just check-requirements which calls uv pip list --outdated

   This is for previewing what the next command may update or not...

   (See note on pinning packages below.)

   • If a version bump is within a minor release (e.g. django 4.2.19 to django 4.2.20), we don't need to manually change anything, the next step will upgrade this.
   • If a version bump includes a minor or major version number, we update the pyproject.toml file if we want this.

2. just compile-requirements which calls uv lock --upgrade

   This actually updates the uv.lock file with the latest updates. Diffing this is a little less scannable than requirements.txt files but okay.

Note on pinning packages

I'd also like to point out the pyproject.toml's format of dependencies, using ~=. For example django~=5.2 will allow any dot releases, but not anything above 5.2.x. This is nicer than django>=5.2 as django 6.0 would qualify once that got released. Most dependencies I've tried to use this. If we need to hard pin we can do that with == in the pyproject.toml still.