Ignore flake8 warning W504

Currently breaking tests with the latest flake8, which added a pair of competing rules, W503 and W504, for whether line breaks should be before or after a binary operator. `ignore` needs to ignore at least one of them, otherwise `flake8` can never be pleased.

Author: adamchainz

setup.cfg

@@ -2,7 +2,7 @@
 tag_svn_revision = false
 
 [flake8]
-ignore = W601               ; # noqa doesn't silence this one
+ignore = W504, W601
 max-line-length = 100
 
 [isort]