nbdev upgrade breaks CI

[mschwoer]

Describe the bug nbdev had a major rewrite (https://github.com/fastai/nbdev/releases/tag/2.0.0) including breaking changes, such that the CI pipeline does not work anymore. As the requirements.txt has nbdev>=1.1.22, this latest version is pulled.

To Reproduce see https://github.com/MannLabs/alphapept/runs/7787447345?check_suite_focus=true

Additional comments don't forget to update the docu :-) https://github.com/MannLabs/alphapept/blob/master/docs/contributing.html