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.
Which must be done by saying::

    from unittest import mock

...because if you say::

    import mock

...you definitely will not be getting the standard library mock.
That will always import the third party mock library.

This commit adds hacking check N366 to enforce the former.

This check can be removed in the future (and we can start saying
``import mock`` again) if we manage to purge these transitive
dependencies. I'm not holding my breath.

[1]https://review.opendev.org/#/c/717540

Change-Id: I8c8c99024e8de61d9151480d70543f809a100998

tox.ini

@@ -105,6 +105,7 @@ extension =
    N340 = checks:check_oslo_i18n_wrapper
    N341 = checks:check_builtins_gettext
    N342 = checks:no_redundant_import_alias
+   N366 = checks:import_stock_mock
 paths = ./watcher/hacking