Consider enforcing flake8, black and isort

As part of #398 I've added disabled calls to flake8 including a commented-out installation of the flake8-black and flake8-isort plugins.

I would suggest to run black and isort once over the code base and enabling both plugins to have a positive long-term effect on code quality. I didn't make this part of #398 since it's not a hard requirement for Jazzband projects (yet) but I've seen more and more projects move to that strategy.