CI: Add coverage diff reporting

[thekaveman]

Using https://github.com/marketplace/actions/python-coverage-comment

[github-actions]

Coverage report

Note
No coverage data of the default branch was found for comparison. A possible reason for this is that the coverage action has not yet run after a push event and the data is therefore not yet initialized.

The coverage rate is 97.29%.
The branch rate is 95%.

None of the new lines are part of the tested code. Therefore, there is no coverage data about them.