CI: codecov for pyx files
#### Is your feature request related to a problem?

cython supports [line coverage](https://cython.readthedocs.io/en/latest/src/tutorial/profiling_tutorial.html#enabling-coverage-analysis) but pyx files are currently not included in the codecov reports. Having code coverage for cython code might help to remove unused cython code.

#### Describe the solution you'd like

pyx files are included in the codecov report.