Finalize the automated tasks in nox

- only support Python 3.8 in the project
  as GitHub Actions has no 3.9 in its base image
- re-order some sessions so that the order in
  `poetry run nox --list` is more intuitive
- add extensive documentation to the noxfile.py module
- add more whitespace to noxfile.py to make it easier to read
- add generic maintainance tasks:
  + "init-project" => set up all pre-commit hooks
  + "clean-pwd" => ~ `git clean -X` with minor exceptions
- upgrade isort to 5.3.0