add parallel build coverage and cleanup CI

[Zeitsperre]

Pull Request Checklist:

• [ ] This PR addresses an already opened issue (for bug fixes / features)

  • This PR fixes issue #xyz

• [x] Tests for the changes have been added (for bug fixes / features)

• [ ] Documentation has been added / updated (for bug fixes / features)

• [x] HISTORY.rst has been updated (with summary of main changes)

• [x] I have added my relevant user information to AUTHORS.md

• What kind of change does this PR introduce?:

• adds the coveralls coverage service to the CI

• cleans up the tox configuration a bit

• Does this PR introduce a breaking change?:

No.

• Other information: