Enforce "black" code style #870

related to #294

This seems to become more an more an issue: this project does not enforce a code formatting style, but in the Python ecosystem there is now a pronounced gravity towards black/ruff format.
A lot of people (myself included) have set up their development environment to automatically apply this code style (or at least partially), but this causes issues with signal/noise ratio in PRs.

I have been reluctant to do a big bang reformat of the code, as this would break all ongoing pull requests,
but lately I've noticed that not fully following the black code style is causing more problems.