Use black for a big code clean-up

Black (https://github.com/ambv/black) is the uncompromising Python code formatter, a newcomer that has been adopted by big projects like requests and pytest.

The particularity of this tool is to format the code without breaking anything: it will check the compiled bytecode to ensure no behavior changes.

The goal is to launch the tool one time on the entire code base and check if a GitHub hook can be used to auto-format pull requests.