Correct lint errors and silence W504

[emmatyping]

Flake8 3.6.0 is a bit more strict on some things, so this fixes the complaints and silences W504 (line break after operator) since that is very common in our code (and we seem to silence W503 too).

For an example of what the failing flake8 output looks like, see https://travis-ci.org/python/mypy/jobs/445480219.

[JukkaL]

Thanks for cleaning up the lint errors! I particularly like that we can now catch missing spaces around = in argument default values.