Use `tox` for testing and add coverage and flake8

Philipp Markwardtrequested to merge
feature/tox into main
  • tox seems to be helpful to execute test. So using this tool as default and removing test splitting
  • Adapt Makefile using tox, remove splitting the test and just keep test target. The splitting is now done by tox
  • Also test using flake8, remove all ignored cases but E203 (this is neccasary for black formating, there would be problems)
  • Add stuff created by new tooling to .gitignore
  • Adapt .gitlab-ci.yml, explicit add coverage badge regex for pytest.

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