Fix error check in test_edit.py for python 3

This isn't the nicest of checks... we should probably be checking the
actual form passed into the context.  But for now, it's a fix.

mediagoblin/tests/test_edit.py

@@ -250,5 +250,5 @@ class TestMetaDataEdit:
         old_metadata = new_metadata
         new_metadata = media_entry.media_metadata
         assert new_metadata == old_metadata
-        assert ("u'On the worst day' is not a 'date-time'" in
+        assert (b"'On the worst day' is not a 'date-time'" in
             response.body)