Use the Codecov CLI to upload coverage

[matthewfeickert]

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Summary

To potentially use Sentry's/Codecov's ATS we first need to use the Codecov CLI to upload our coverage reports.

https://docs.codecov.com/docs/the-codecov-cli#how-to-upload-to-codecov makes it seem like the workflow would be

$ python -m pip install --upgrade codecov-cli
$ codecovcli create-commit
$ codecovcli create-report
$ codecovcli do-upload

though I'm not sure how to replace all the options we use with the codecov-action

https://github.com/scikit-hep/pyhf/blob/fb604e89c274aabfc3435e2e0efd504b549b51f3/.github/workflows/ci.yml#L64-L72

Additional Information

CLI Options: https://docs.codecov.com/docs/cli-options

Code of Conduct

• [X] I agree to follow the Code of Conduct

[matthewfeickert]

Given https://docs.codecov.com/docs/cli-options this might be doable with just

$ python -m pip install --upgrade codecov-cli
$ codecovcli create-commit
$ codecovcli create-report
$ codecovcli do-upload --file ./coverage.xml --flag unittests-${{ matrix.python-version }}