Pass reason to `@skip` decorator

[mthuurne]

The @skip decorator works without an argument as well, but that is an undocumented feature of unittest.skip() that is not understood by mypy and pytest.

In the case of pytest, it ignored the decorated class during test collection, instead of collecting it and marking it as skipped.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.57%. Comparing base (1e481c4) to head (ab8a8ae).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #595 +/- ## ======================================= Coverage 98.57% 98.57% ======================================= Files 6 6 Lines 773 773 ======================================= Hits 762 762 Misses 11 11 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.