Move to pytest-cov

[mstimberg]

We previously used the separate coverage.py utility, which seems not to handle our somewhat convoluted pytest setup 100% correctly – I did not look into it in much detail, but our coverage on coveralls clearly showed code as not covered that was covered by tests (I double-checked by raising an Exception in the "uncovered" code). Using pytest-cov seems to be the better solution, and while there were a few issues with multiprocessing (since we also run our tests in parallel), it now seems to run smoothly. Without adding a single test, our test coverage jumps from 82% to 92% 😆