Notebook linting

[pnkraemer]

Is your feature request related to a problem? Please describe.
ProbNum's source is linted rather extensively, but the notebooks are not at all.
This leads to some discrepancies in code quality, which are not great, especially since it affects the tutorials.

Describe the solution you'd like.
As discussed offline, we could apply black, isort, and pylint to the notebooks via nbqa https://github.com/nbQA-dev/nbQA as part of the CI. This would detect some basic issues with the code quality in jupyter notebooks before merging them into main.

Additional context

• As also discussed offline, it might not be a bad idea to skip the pre-commit hook (at least for the beginning), because when committing changes to a notebook while having the notebook opened, the file contents get overwritten and jupyter runs into some issues.
• Implementing this could involve a PR with a large diff, because almost all notebooks would presumably be affected.
• It could make sense to implement this in 2 steps: black and isort could be done straightforwardly, pylint will require a large number of --disable="..." statements and/or nontrivial refactoring in the notebooks.

[marvinpfoertner]

black supports notebook formatting out-of-the-box now: https://github.com/psf/black/releases/tag/21.8b0.
We simply need to add black[jupyter] in format-requirements.txt and everything should work without needing to modify anything else :)