Replace codecov for simpler GitHub Actions solution

[santisoler]

Replace push of test coverage reports to codecov for a simpler solution that publishes coverage report in GitHub Actions outputs. Remove .coveragerc and move configs to pyproject.toml. Run coverage on tests as well.

Relevant issues/PRs:

Inspired on changes made in fatiando/bordado.

Todo

• [ ] Open issue to generate HTML report in case of failure. This might be tricky due to the flat layout we are using in Harmonica.
• [ ] Tell coverage to ignore the functions that are decorated with @run_only_with_numba.

[leouieda]

@santisoler I got this working in Tremelique today and I implemented the fix here (hopefully it works).

[leouieda]

Not sure why it's still failing here. Will investigate.

[santisoler]

Closing this one for now... Last month I sorted out codecov failures. I think we can keep it for a while longer.