tests: Use version specific import for mock

Signed-off-by: Jussi Kukkonen <jkukkonen@vmware.com>

Author: Jussi Kukkonen

tests/test_indefinite_freeze_attack.py

@@ -45,7 +45,6 @@
 from __future__ import unicode_literals
 
 import datetime
-import unittest.mock as mock
 import os
 import time
 import tempfile
@@ -55,6 +54,11 @@
 import unittest
 import sys
 
+if sys.version_info >= (3, 3):
+  import unittest.mock as mock
+else:
+  import mock
+
 import tuf.formats
 import tuf.log
 import tuf.client.updater as updater