Coding style standard

[frafra]

Is your feature request related to a problem? Please describe.
Coding style standard is inconsistent.

Describe the solution you'd like
black is a great tool to reformat your code. Together with pre-commit can assure that every commit follows the rules. A badly formatted modified file make impossible to commit, because the file gets changed/reformatted during the commit phase. Adding the file again will make commit work again.

This mechanism works only if pre-commit is installed on the client (those are client-side hooks).

Describe alternatives you've considered
flake8 is just a checker and we could spare dev time by just use a code formatter.
isort is also nice, but it is for imports only and it would be more a nice additional hook than a replacement for black.

Additional context
I am not proposing to reformat everything at once: that would break git history. Giving a nice tool to help developers respecting code style rules for new commit could improve things over time.

This change would preferably require to update the documentation too.