DEP: make pyproject.toml the one source of truth for test-only dependencies

[neutrinoceros]

This PR does 3 things:

One source of truth

Dependency groups

Move dev-only dependency specifications from tox.ini to pyproject.toml as PEP 735 Dependency Groups.

The PEP was accepted a couple weeks ago and is already supported by tox as well as uv, which allow running tests without tox as, for instance

uv run --group test pytest

Rely on uv's dependency resolver to test with pinned versions

Avoid duplication of metadata for oldest supported versions of our dependencies by switching dependency resolution strategy. This is supported by uv (via tox-uv).
Notes:

• this also means that test dependencies (e.g. pytest) are pinned as well (good)
• using this strategy requires setting explicit minimal requirements everywhere (otherwise one gets the oldest version in existence, which in a very general way is completely broken)
• in order to properly test this on my setup (Apple Silicon), I needed to add a layer of requirements specifically for this platform (as our absolute-oldest supported versions work on ubuntu but are generally lacking support for Apple Silicon)

Drop pytest-cov

I removed pytest-cov from our testing pipeline, as it's a completely unnecessary layer of indirection between pytest and coverage. I switched to direct invokes to coverage.

While I was at it, I also re-organized tox.ini a bit (mostly moving lines around).

[neutrinoceros]

Apparently something I did broke tox env selection in CI so every job runs every env. I don't know yet if that's the reason it's failing, or if that's a different problem.

[neutrinoceros]

I resolved the problem mentioned above by using tox-gh (first party plugin) instead of tox-gh-actions (third party plugin). This could probably be avoided or simplified but I already spent > 3hrs on this so I'll quit while I'm ahead.

[neutrinoceros]

an interesting side effect of using tox-uv in CI is that it saves a surprising amount of CPU time.
See for instance usage pages

• before (total CPU time: 16m23s)
• after (total CPU time: 11m2s)