Use flake8's extend-ignore instead of ignore

extend-ignore adds ignores in addition to flake8's existing ignores.

The default ignores currently are:
E121,E123,E126,E226,E24,E704,W503,W504

Author: bluetech

tox.ini

@@ -156,7 +156,7 @@ markers =
 
 [flake8]
 max-line-length = 120
-ignore = E203,W503
+extend-ignore = E203
 
 [isort]
 ; This config mimics what reorder-python-imports does.