Update pytest-mock to 3.1.1

[pyup-bot]

This PR updates pytest-mock from 3.1.0 to 3.1.1.

Changelog

3.1.1

------------------

* Fixed performance regression caused by the ``ValueError`` raised
when ``mocker`` is used as context manager (`191`_).

.. _191: https://github.com/pytest-dev/pytest-mock/issues/191

Links

• PyPI: https://pypi.org/project/pytest-mock
• Changelog: https://pyup.io/changelogs/pytest-mock/
• Repo: https://github.com/pytest-dev/pytest-mock/

[pyup-bot]

Closing this in favor of #61