Blacken uses black to format a Python buffer. It can be called explicitly on a
certain buffer, but more conveniently, a minor-mode 'blacken-mode' is provided
that turns on automatically running black on a buffer before saving.
To automatically format all Python buffers before saving, add the function
blacken-mode to python-mode-hook:
(add-hook 'python-mode-hook 'blacken-mode)
