Calculate code coverage

[ModischFabrications]

https://realpython.com/pytest-python-testing/

[ModischFabrications]

Replace #20 with this!

[ModischFabrications]

https://pytest-cov.readthedocs.io/en/latest/ from github actions could be kind of nice

[ModischFabrications]

also see pytest-dev/pytest-cov#374 to eventually skip generation of useless .coverage file