Publish test coverage in CI

[uniqueg]

@kellrott: Server-side this already seems to be set up for Coveralls. However, when migrating our services and tools to GitHub Actions from Travis CI, we had a hard time getting Coveralls to work and migrated our services to Codecov instead. I could try again to make it work with Coveralls if you like (it's a little while ago) - but otherwise Codecov has worked really nice for us.

In that case, you would have to set this up on the Codecov server though, @kellrott, and set a repository secret. And then replace the last task in the CI workflow in the 0.5.0 release branch with the following two (you may of course need to change the name of the secret if you did not use CODECOV_TOKEN):

      - name: Run unit tests
        run: |
          coverage run --source tes -m pytest -W ignore::DeprecationWarning
          coverage xml
      - name: Submit coverage
        uses: codecov/codecov-action@v3
        with:
          token: ${{ secrets.CODECOV_TOKEN }}
          files: ./coverage.xml
          fail_ci_if_error: true
          verbose: true

Either way, please let me know.