Incorporate coverage diff reporting

[thekaveman]

We use the coverage package to calculate Python unit test coverage of the codebase.

When the tests are run in CI, we generate a coverage report, but this doesn't go anywhere and isn't a very useful output for us.

Within a given PR or set of changes, we want more 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 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

Corrollary to cal-itp/benefits#1668

Example of this setup