flake8: ignore outdated rule

W503 is an outdated warning which contradicts modern W504 [1].

1. https://www.flake8rules.com/rules/W503.html

Part of #270

Author: DifferentialOrange

.flake8

@@ -1,3 +1,3 @@
 [flake8]
 # Use pylint for lines length check
-ignore=E501
+ignore=E501,W503