Downgrade black, fix removed imports from __init__.py

[timonegk]

• Unused imports were removed but are required for better import from other packages. This PR changes it to use __all__ convention
• Downgrade black for Ubuntu's pre-commit version (confine pre-commit to stages psf/black#3940 is not supported, wait until Ubuntu updates pre-commit or Fix support for pre-commit versions < 3.2.0 psf/black#4041 is merged)

[jaagut]

How is pre-commit installed on your system? I have it via pip (see requirements/dev.txt). With that, I did not notice any black version issues