Add pyflakes option to tox

* Uses pyflakes wrapper to ignore gettext errors
* Based on: https://bugs.launchpad.net/pyflakes/+bug/844592

Change-Id: I4fd947176f5258f89f517acf5dda2ded4a2da15e

flakes.py

@@ -0,0 +1,22 @@
+import __builtin__
+import os
+import sys
+
+from pyflakes.scripts.pyflakes import main
+
+"""
+ wrapper for pyflakes to ignore gettext based warning:
+     "undefined name '_'"
+
+ From https://bugs.launchpad.net/pyflakes/+bug/844592
+"""
+
+names = os.environ.get('PYFLAKES_BUILTINS', '_')
+names = [x.strip() for x in names.split(',')]
+for x in names:
+    if not hasattr(__builtin__, x):
+        setattr(__builtin__, x, True)
+
+del names, os, __builtin__
+
+sys.exit(main())