flake8 config: ignore W503

[AlexWaygood]

This rule contradicts PEP8, which now says that you should write multiline expressions like this:

if (
    foo
    and bar
):

When W503 was originally introduced, PEP8 had a different recommendation:

if (
    foo and
    bar
):

This PR is needed for #132, which has some lines that go against W503.