Use unittest.mock instead of PyPI mock

Now that py2 is gone, oslotest dropped dependency on mock and will
soon affect Ussuri CI [1], let's use unittest.mock built in py3.

This also fixes py38 jobs and proactively prevents py36 and py37
failing due to [1]. This is because we never included mock in
test-requirements (but in lower-constraints where it does not
really belong at all) and instead relied on oslotest to bring
it in.

[1] https://review.opendev.org/716322

Change-Id: I30e82e2d87418272a71c7ee089a8acdaf8872158