Incorporate coverage diff reporting

[thekaveman]

We use the coverage package (indirectly via pytest-cov) to calculate Python unit test coverage of the codebase.

The test runner generates a coverage report, the latest always available on our docs site at: https://docs.calitp.org/benefits/tests/coverage

However, within a given PR or set of changes we don't get much insight of the impact on test coverage -- without manually reviewing coverage reports and eyeballing differences.

The Python Coverage Comment GitHub Action can be used in a workflow to provide coverage diffs for our pytests. The action uses existing coverage metadata generated by the test runner to create a PR comment with information about the change. The comment is updated as the PR changes and new test runs are produced.

Example: comment on a first-time setup PR

Acceptance Criteria

• Use coverage directly rather than via pytest-cov
• Use pyproject.toml to configure coverage
  • See note about running with relative_files = true
• Update pytest actions workflow to use Python Coverage Comment to provide coverage diff reporting, example

Additional context

Example of this setup

[machikoyasuda]

This was completed with #1745 right?