Enable auto formatting

This will introduce both Black and isort. As part of adding Black, a few of the previous Flake8 plugins are being removed and Bugbear is being added.

Most of the current lint settings are being left alone -- the most noticeable exception is line length, which is being reduced by two characters to match Black's default -- while being moved to setup.cfg to make them accessible outside of tox.

Closes #49 (closed)