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>

(cherry picked from commit ff5a7e6377)

setup.py

@@ -66,5 +66,5 @@ setup(
         "dogpile.cache",
     ],
     extras_require={':python_version=="2.7"': ["enum34", "lockfile"]},
-    tests_require=["mock", "pytest", "pytest-cov", "pyfakefs"],
+    tests_require=["pytest", "pytest-cov", "pyfakefs"],
 )

test-requirements.txt

@@ -1,4 +1,4 @@
-mock
+mock; python_version < '3.3'
 parameterized
 pytest
 pytest-cov

tests/test_pkgset_pkgsets.py

@@ -1,4 +1,5 @@
 # -*- coding: utf-8 -*-
+
 import ddt
 from unittest import mock
 import os

tests/test_pkgset_source_koji.py

@@ -1,7 +1,8 @@
 # -*- coding: utf-8 -*-
 
 import json
-
+import unittest
+from unittest import mock
 import os
 import re
 import six
@@ -10,13 +11,6 @@ from typing import AnyStr, List, Set, Dict, Tuple
 
 from tests.test_gather_method_hybrid import MockModule
 
-try:
-    import unittest2 as unittest
-    from unittest2 import mock
-except ImportError:
-    import unittest
-    from unittest import mock
-
 from pungi.phases.pkgset.sources import source_koji, source_kojimock
 from tests import helpers
 from pungi.module_util import Modulemd