Code coverage reports in CI pipeline

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Add code coverage reports in the GitHub actions test workflow.

The minimum should be a textual code coverage report in the test output (so basically a tox -e report). Ideally though, an HTML coverage report would be nice.

An option would be to use a service like https://coveralls.io/.

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Coverage reports are now part of the unit test text output (#65).

A nice HTML coverage report would still be nice though. :)