Add test coverage with github actions

[SabineHaas]

When changing to gibhub actions from travis CI the code coverage stayed at travis, which is why it was not updated since December.
We should implement the code coverage with github actions now.

[SabineHaas]

Note, to add threshold for failing check at < specific test coverage:

- name: Check test coverage
      run: coverage report -m --fail-under=${{ matrix.vcs == 'bzr' && 89 || 90 }}

[SabineHaas]

Open todo after merge of this PR (#283):

• adapt badge in README.rst

Originally posted by @SabineHaas in #283 (comment)