Merge pull request matplotlib#2217 from AdamHeck/master

Fix false detection of gtk3 bindings during install

setupext.py

@@ -1473,7 +1473,7 @@ def check(self):
             raise CheckFailed("Can't build with Travis")
 
         if sys.version_info[0] >= 3:
-            return "gtk3agg backend does not work on Python 3"
+            raise CheckFailed("gtk3agg backend does not work on Python 3")
 
         # This check needs to be performed out-of-process, because
         # importing gi and then importing regular old pygtk afterward