Fix test_gettextutils on Python 3

On Python 3, str(obj) calls obj.__str__(), not obj.__unicode__().

The test does not pass with nosetests, LogLevelTranslationsTest fails with an
AttributeError (self.level is missing), because nosetests doesn't play well
with test scenarios.

Change-Id: I78ae106e336480d9995ac4394197ea45b8ef9b07
Merge "Fix test_gettextutils on Python 3"