Parallelize Test Workflows

[LarsKue]

We can parallelize the test workflow in 2 ways:

1. Over multiple environments (i.e., python versions). This can be done either via parallelization directly in the github workflow, or we can tell tox to parallelize environments.
2. Over multiple tests with pytest-xdist using the flag -n auto.

Drawbacks:

• Parallelizing the tests changes the console output for the test workflow, since intermediate results cannot be shown until all tests have completed.

[LarsKue]

Done! Tests now run in approximately 8 instead of 18 minutes :partying_face:

[stefanradev93]

That's awesome!