hacking: force explicit import of python's mock

Since we dropped support for python 2 [1], we no longer need to use the
mock library, which existed to backport py3 functionality into py2.
Change Ib44b5bff657c8e76c4f701e14d51a4efda3f6d32 cut over to importing
the stock mock, which must be done by saying::

    from unittest import mock

...because if you say::

    import mock

...you will be using the third party mock library instead, which may or
may not be installed.

This commit adds hacking check N371 to enforce the former.

[1] https://review.opendev.org/#/c/687954/

Change-Id: I71439580e80d33cff62aba807df2b35164a47cbe

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