I can't tell right now cuz all the tests are failing, but switching from pytest-cov to coverage will reduce coverage if we've got any tests running in subprocesses, unless we set up implicit coverage.

I was just trying to solve the "no coverage" found issue on the min jobs (first commit). Looks like pytest-xdist might cause problems too. Okay, let's just try to fix pytest-cov.

Okay, working, here's a summary of the changes:

Min version test:

• Pinned setuptools_scm for older Pythons as it now errors out with old setuptools
• Increased the pin on packaging on older pythons to make setuptools_scm happy
• Used a simpler way to set the constraints in tox but still have them cleared for subprocess calls

I also had to remove the error on coverage combine, as it errors out when there's nothing to combine, and that happens on the min version test since we only do one run in CI.

I also added pip to the deps, as it's required when using tox-uv (which is what I'm using locally).

I think there's an issue with threading where setuptools may fight over the same source directory from multiple threads, causing an occasional error we've seen for a while. But not fixing that here.