Add missing dependency on pytest-mock

For some reason, tox installs it on Jenkins, but not on my computer.

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Migrated from D5666 (view on Phabricator)

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Patch application report for D5666 (id=20234)

Rebasing onto d7c3a7e1...

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commit 30b40cce96e32205ba81ea7df0d5437175d9039f
Author: Valentin Lorentz <vlorentz@softwareheritage.org>
Date:   Tue May 4 10:32:56 2021 +0200

    Add missing dependency on pytest-mock
    
    For some reason, tox installs it on Jenkins, but not on my computer.

See https://jenkins.softwareheritage.org/job/DTSCN/job/tests-on-diff/123/ for more details.

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