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_pkgset_pkgsets.py

@@ -4,7 +4,10 @@
 import mock
 import os
 import sys
-import unittest
+try:
+    import unittest2 as unittest
+except ImportError:
+    import unittest
 import json
 import tempfile