Add commas in our error ignore so autopep8 understands them

See hhatto/autopep8#708. Previously running
autopep8 would move our imports around.

Author: PeterJCLaw

setup.cfg

@@ -10,11 +10,11 @@ exclude =
     venv
 ignore =
     # don't require set comprehensions
-    C401
+    C401,
     # Ignore flake8's complaints about import positions since we're using isort
-    E402
+    E402,
     # W503 and W504 conflict; ignore the one that is less visually appealing.
-    W504
+    W504,
 
 per-file-ignores =
     # This is the example file, where we encourage use of `import *`.