Use black

[dcrosta]

(Mostly) replace the flake8 stuff with black. We'll want to keep flake8 for import sorting. See https://gitlab.com/bigapplepy/yak-bak/blob/master/tox.ini#L41-49 and https://gitlab.com/bigapplepy/yak-bak/blob/master/tox.ini#L58-67 for reference on what we want to do here.