Make python3-mock dependency optional

https://fedoraproject.org/wiki/Changes/RemovePythonMockUsage

Prefer using unittest.mock to a standalone package. The separate
packages should only really be needed on Python 2.7 these days.

The test requirements file is updated to only require mock on old
Python, and the dependency is removed from setup.py to avoid issues
there.

Relates: https://src.fedoraproject.org/rpms/pungi/pull-request/9

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

tests/test_unified_isos.py

@@ -1,6 +1,9 @@
 # -*- coding: utf-8 -*-
 
-import mock
+try:
+    from unittest import mock
+except ImportError:
+    import mock
 import os
 import shutil
 import six