tests: fix mixed imports of 'unittest.mock' and 'mock'

The 'mock' package is imported in 'tests.base'. Whenever possible
'tests.base' prefers 'unittest.mock' over 'mock' (which is the case on
Python 3.3+). Importing 'mock' together with 'unittest.mock' causes
unexpected behavior that results in exceptions when running tests, at
least on Python 3.6.12, like:

>  File "/usr/lib/python3.6/unittest/mock.py", line 1196, in patched
>    patching.__exit__(*exc_info)
>
>  File "/home/zuul/src/opendev.org/jjb/jenkins-job-builder/.tox/cover/lib/python3.6/site-packages/mock/mock.py", line 1545, in __exit__
>    return exit_stack.__exit__(*exc_info)
>
>  File "/usr/lib/python3.6/contextlib.py", line 339, in __exit__
>    received_exc = exc_details[0] is not None
>
>  IndexError: tuple index out of range

From now on, to avoid similar issues in the future, the 'mock' package
is only installed for Python 2.x.

Change-Id: Ib3044312f0770735d88d8657d1f953e5a33e771f

test-requirements.txt

@@ -10,5 +10,5 @@ testscenarios>=0.4 # Apache-2.0/BSD
 testtools>=1.4.0 # MIT
 stestr>=2.0.0,!=3.0.0 # Apache-2.0/BSD
 tox>=2.9.1 # MIT
-mock>=2.0 # BSD
+mock>=2.0; python_version < '3.0'  # BSD
 sphinxcontrib-programoutput

tests/cmd/test_config.py

@@ -1,13 +1,14 @@
 import io
 import os
 
-from mock import patch
 from tests.base import mock
 from tests.cmd.test_cmd import CmdTestsBase
 
 from jenkins_jobs.cli import entry
 from jenkins_jobs import builder
 
+patch = mock.patch
+
 
 @mock.patch("jenkins_jobs.builder.JenkinsManager.get_plugins_info", mock.MagicMock)
 class TestConfigs(CmdTestsBase):

tests/multibranch/test_multibranch.py

@@ -14,7 +14,7 @@
 # under the License.
 
 from tests import base
-import mock
+from tests.base import mock
 import os
 from jenkins_jobs.modules import project_multibranch