Configure pytest-mock as pytest required-plugin

[bjlittle]

🚀 Pull Request

Description

This pull-request configures the pytest-mock plugin for pytest, thus making the mocker fixture available.

See https://pytest-mock.readthedocs.io/en/latest/ for further details.

Note that, through the mocker fixture, we have access to vanilla mock module as we know and (un-)love.

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[ESadek-MO]

LGTM! Ta @bjlittle