Bug 1473498 - [mach] Enable test_config with Python 3 r=firefox-build-system-reviewers,chmanchester

Depends on D36100

Differential Revision: https://phabricator.services.mozilla.com/D36101

--HG--
extra : moz-landing-system : lando

python/mach/mach/test/python.ini

@@ -4,7 +4,6 @@ subsuite = mach
 [test_conditions.py]
 skip-if = python == 3
 [test_config.py]
-skip-if = python == 3
 [test_dispatcher.py]
 [test_entry_point.py]
 [test_error_output.py]

python/mach/mach/test/test_config.py

@@ -140,7 +140,8 @@ class TestConfigSettings(unittest.TestCase):
         a = s.a
 
         # Assigning an undeclared setting raises.
-        with self.assertRaises(AttributeError):
+        exc_type = AttributeError if sys.version_info < (3, 0) else KeyError
+        with self.assertRaises(exc_type):
             a.undefined = True
 
         with self.assertRaises(KeyError):