Use black for formatting

[Stefan Appelhoff]

should be done in a single, squashed commit, and then ensured via CI (e.g., flake-black).

We then use:

• isort
• black
• flake8 (with numpydoc docstr checks)

For example as done here:

• (optional!) pre-commit: https://github.com/sappelhoff/pyprep/blob/master/.pre-commit-config.yaml
• CI tests: https://github.com/sappelhoff/pyprep/blob/038c593bfab0f3021f5b1a037d542a4f29189eb4/.github/workflows/python_tests.yml#L63-L70