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

https://flake8.pycqa.org/en/latest/user/error-codes.html
https://github.com/PyCQA/pyflakes/blob/master/NEWS.rst

I've checked with a variety of versions of flake8 (/pyflakes) and
they all pass.