Coverage reporting refactor and cleanup

[Cadair]

fixes #395 and fixes #383

This PR makes the following changes:

1. Breaking change you need to set setenv = COVERAGE_FILE={toxinidir}/.coverage in tox.ini if your tests are not run in the {toxinidir} directory, so that reporting finds the .coverage file.
2. coverage: codecov no longer requires the use of pytest-cov, the xml coverage report is generated using coverage.py in a separate step to the tox run.
3. We now support OIDC auth to codecov meaning you don't need CODECOV_TOKEN if using this.
4. coverage: github is now tested against both pytest-cov and coverage.py, and as long as the .coverage files are in the root directory will work correctly.
5. As discussed in BUG: Coverage that works for v2.4.0 breaks after v2.5.0 #383 (comment) codecov will now fail the job on error, including no files updated. This is particularly critical with the breaking change.

[codecov-commenter]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️

[astrofrog]

This seems ok to me! Did you test it out on a real package? If so feel free to merge.

[Cadair]

I'm having trouble with it on sunpy: sunpy/sunpy#8665

[Cadair]

Fixed sunpy: https://github.com/sunpy/sunpy/actions/runs/26883832859

[Cadair]

Do people think it's worth throwing a warning about the COVERAGE_FILE change if we get to the codecov upload step and there isn't a .coverage file to work with?

[pllim]

Do people think it's worth throwing a warning

I would say, yes. Not everyone follows the PRs here or even read the change log.

[Cadair]

Ok, done, and failing test added to demo. I think this is ready, I'll put out a 3.0 after this.

[pllim]

I have a comment and a question.

[pllim]

So you just gonna leave the py314-tmpdir_warning job red?

[Cadair]

So you just gonna leave the py314-tmpdir_warning job red?

yep lol

[pllim]