This adds support for flake8 through Review Bot, which is just a combination of

other, already available tools (pycodestyle and pyflakes).
While I was here I noticed the description for one of the pycodestyle options

was incorrectly referencing the old pep8 tool, and the filename parsing was

fragile in the same way that flake8 would be, so I've fixed that up.

Configured the tool and saw it run successfully.