chore(python): add E231 to .flake8 ignore list (#1379)

Author: parthea

synthtool/gcp/templates/python_library/.flake8

@@ -16,7 +16,7 @@
 
 # Generated by synthtool. DO NOT EDIT!
 [flake8]
-ignore = E203, E266, E501, W503
+ignore = E203, E231, E266, E501, W503
 exclude =
   # Exclude generated code.
   **/proto/**