Ignore W504 in flake8

PEP 8 recommends to break a long line after a binary operator. That's
used in the current code.

In the past PEP 8 wasn't so specific and W504 appeared in flake8. This
one makes flake8 ignore W504.

For details see PyCQA/pycodestyle#498

tox.ini

@@ -23,7 +23,8 @@ commands =
 # D103: Missing docstring in public function
 # D104: Missing docstring in public package
 # D401: First line should be in imperative mood; try rephrasing
-ignore = D100, D101, D102, D103, D104, D401
+# W504: line break before binary operator
+ignore = D100, D101, D102, D103, D104, D401, W504
 max-line-length = 100
 exclude =
     .tox,