Use unittest2 if available

The module backports features to Python 2.6 and 2.7. If it is available,
the tests will use it. If it is not available, it will fall back to
regular unittest. On Python 2.7, the tests pass anyway. On Python 2.6,
there are failures with Python 2.6.

Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>

tests/test_createiso_phase.py

@@ -2,7 +2,10 @@
 # -*- coding: utf-8 -*-
 
 
-import unittest
+try:
+    import unittest2 as unittest
+except ImportError:
+    import unittest
 import mock
 
 import os