Update flake8 to 3.7.4

[pyup-bot]

This PR updates flake8 from 3.7.3 to 3.7.4.

Changelog

3.7.4

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

You can view the `3.7.4 milestone`_ on GitLab for more details.

Bugs Fixed
~~~~~~~~~~

- Fix performance regression with lots of ``per-file-ignores`` and errors
(See also `GitLab!299`_, `GitLab501`_)


.. all links
.. _3.7.4 milestone:
 https://gitlab.com/pycqa/flake8/milestones/27

.. issue links
.. _GitLab501:
 https://gitlab.com/pycqa/flake8/issues/501

.. merge request links
.. _GitLab!299:
 https://gitlab.com/pycqa/flake8/merge_requests/299

Links

• PyPI: https://pypi.org/project/flake8
• Changelog: https://pyup.io/changelogs/flake8/
• Repo: https://gitlab.com/pycqa/flake8