use black to automatically format the code

[carinedengler]

Is your feature request related to a problem? Please describe.
automatically formatting the code

• allows the developer to concentrate on the code's content, not its format
• makes the code easier to maintain
• makes the code more readable

Describe the solution you'd like
use black to automatically format the code (for example, see francearchives-cubicweb) and integrate a git hook which formats the code automatically on commits

Describe alternatives you've considered
make the code PEP8 conform and integrate linting step (flake8) into Travis CI

[PonteIneptique]

No issue on my end, only obligation would be 120 line char (I hate the 88 limitation) and 4 spaces (but I think it's generally the default)