Drop unittest2

The library is imported if available, but we never build it in any
environment where the package would be installed. It was last used for
RHEL 6 builds.

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

tests/test_imagechecksumphase.py

@@ -1,17 +1,14 @@
 # -*- coding: utf-8 -*-
 
-try:
-    import unittest2 as unittest
-except ImportError:
-    import unittest
 try:
     from unittest import mock
 except ImportError:
     import mock
 
 import os
-import tempfile
 import shutil
+import tempfile
+import unittest
 
 from pungi.phases.image_checksum import ImageChecksumPhase, dump_checksums
 from tests.helpers import DummyCompose, PungiTestCase