Prepare CI / CD pipeline for initial release

[ChristophAlt]

• Add flake8 + some plugins for linting of code and docstrings
• Add darglint for argument linting
• Use nox for test automation (run tests against configured python versions and explicitly tests against installed package)
• Remove unittesting from pre-commit
• Make use of poetry mandatory to ensure the built and installed package is tested, not the source code
• Add github workflows for test (code and documentation)
• Add github workflow for release creating
• Support Python 3.8 for now

Although this PR touches quite a few files, most of the changes are either additional configuration files or minor (automated) reformatting of code and other files.

Additional info:

• Currently there are quite a few linting errors, mostly related to missing docstrings.
• One unittest failed, which is due to incorrect use of unittest.approx. This was fixed with this PR.
• I also observed some of the unittests to fail on floating point comparison, this seems quite random (see Don't compare floating point numbers in unittests #74).
• Currently, flake8 is disabled (due to the large number of linting errors). This should be addressed in a subsequent PR (see Enable flake8 in pre-commit checks #87).
• Also, mypy doesn't check for unreachable code (see mypy does not check for unreachable code #86). This should be addressed in a subsequent PR.
• Further, the project structured should be changed to the src/ format. This prevents accidental imports of PyTorch IE (Change project structure to src/ format #75).

[ArneBinder]

As far as I understand the changes, they look good. I have just two minor questions.

Also, could you create one/multiple further issues for the TODOs listed in "additional Info" of the PR description that we do not forget about this? I'm not really into linting/flake8 and don't know if this a single one or separate problems that's why I did not create them myself.