Analogous to D5198, this turns on all the pyflakes checks
in flake8. But we don't need any exceptions because we had
pretty much all F codes listed anyway.
The only ones we didn't have explicitly listed:
F721 syntax error in doctest F722 syntax error in forward annotation F723 syntax error in type comment F901 raise NotImplemented should be raise NotImplementedError
I've checked with a variety of versions of flake8 (/pyflakes) and
they all pass.