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
118ac858c4