Use unittest2 for rhel6 compatibility

Fixes: https://pagure.io/koji/issue/830

tests/test_cli/test_upload_progress_callback.py

@@ -1,7 +1,11 @@
 from __future__ import absolute_import
 import mock
 import six
-import unittest
+try:
+    import unittest2 as unittest
+except ImportError:
+    import unittest
+
 
 from koji_cli.lib import _format_size, _format_secs, _progress_callback