Make Black in CI more user friendly

[PGijsbers]

When files are uploaded which are not Black-formatted, CI gives a less than helpful message about what is wrong. We'll have to look into what is possible, but I would consider the following alternatives:

• Have a bot which lists the Black changes, and you can interact with to have it commit those changes for you (either to identify a bot we want to adopt, or to create one)
• Apply Black formatting if needed and re-commit automatically (is that desirable? By seeing the feedback, at times I get ideas for further refactoring)
• Show the changes in the CI log (maybe there is a Black option, otherwise an added git diff should suffice)

The main use case is to make edits through the web interface smoother. E.g. I had set up this pr through the web to keep it quick, but ended up pulling my branch anyway to see what changes I was required to make to pass pre-commit. It also helps users which have incorrectly configured their pre-commit (we could also have the bot point that out).