tox: disable W504 "Line break occurred after a binary operator"

This is a new coding rule, which actively contradicts the previous
pep8 rule, W503 "line break before binary operator".  Both rules
are currently in force, which doesn't make a lot of sense.

Until the standards people work this out, I'm arbitrarily choosing
to disable the rule that involes the fewest coding changes.

tox.ini

@@ -2,7 +2,7 @@
 envlist = flake8, pylint, yamllint, unit, integration, docs
 
 [flake8]
-ignore = E221,E241,E501
+ignore = E221,E241,E501,W504
 max-line-length = 110
 
 [testenv]