Remove coverage tests from gh workflow.

As github workflows do not support Python2.7, we cannot run coverage
tests (they succeed only, if all Python3 and Python2 parts were tested)
nor Python2.7 tests on github.

We therefore remove both, tests for Python2.7 and coverage tests, from
the github workflow.

The default behavior of the local `tox.ini` still runs these tests and
therefore requires an installed Python2.7 version.