Get mox from mox3, not from six.moves

oslotest 1.7.0 breakes oslo.messaging unittests, because it doesn't add
mox to six.moves anymore - see change Ic59c73abb9b09cb594bf7df4173d7f99f81d526c

This patch fixes imports on test run.

Change-Id: Ia40b7733bd2e74b1a1703f2e1a5245f01debbd36

oslo_messaging/tests/test_transport.py

@@ -14,9 +14,9 @@
 #    under the License.
 
 import fixtures
+from mox3 import mox
 from oslo_config import cfg
 import six
-from six.moves import mox
 from stevedore import driver
 import testscenarios
 

tests/test_transport.py

@@ -14,8 +14,8 @@
 #    under the License.
 
 import fixtures
+from mox3 import mox
 import six
-from six.moves import mox
 from stevedore import driver
 import testscenarios