Fix several problems pointed out by flake8

Fix several problems pointed out by flake8

I really should integrate flake8 more decisively into my workflow somehow.

Note: I'm ignoring W503 here since it's contradictory with W504,
(breaking a "line too long" problem at an operator will cause
one of W503 or W504 to fire and there's no way I can see to shut
them both up).