Add code coverage to the repo

[maxrake]

Description

Add code coverage to the phylum-dev/phylum-ci repository for the phylum-ci package. This is both for computing code coverage and enforcing it.

Additional Details

• Compute the code coverage automatically in a workflow
  • The Test workflow is a likely candidate since it could also be the place to fail the build if coverage drops
  • Provide a report as an artifact...even when the build fails
    • HINT: use the always() logic in GHA for this
  • Fail the build if coverage drops below a set threshold
    • Not sure what is normal here...maybe 80% ???
    • Might also be okay to just ensure coverage does not decrease
      • HINT: use the actions/checkout action, ref input to check both the main branch and the current ref to get a diff
• Expose the ability to compute coverage locally
  • Adding to the test(s) tox environments is a good bet here
• Consider the pytest-cov package/extension for integrating easily with the existing pytest package

Acceptance Criteria

• Code coverage is computed automatically in CI
• Code coverage is available to be computed locally
• Code coverage is enforced in PRs via failing required checks for minimum coverage threshold
• Documentation is updated

[marvin-hansen]

Sample config for automatic code coverage for Python projects:
Note, this requires a CodeCov API token.

https://github.com/codecov/example-python/blob/main/.github/workflows/ci.yml

Documentation for Python Code Coverage with or w/o PR coverage

https://about.codecov.io/blog/python-code-coverage-using-github-actions-and-codecov/

Usually an increase / decrease in code coverage is reported by CodeCov.