Update flake8 to 3.8.4

[pyup-bot]

This PR updates flake8 from 3.8.3 to 3.8.4.

Changelog

3.8.4

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

You can view the `3.8.4 milestone`_ on GitLab for more details.

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

- Fix multiprocessing errors on platforms without ``sem_open`` syscall.  (See
also `GitLab!448`_)

- Fix skipping of physical checks on the last line of a file which does not
end in a newline (See also `GitLab!451`_)

.. all links
.. _3.8.4 milestone:
 https://gitlab.com/pycqa/flake8/-/milestones/37

.. merge request links
.. _GitLab!448:
 https://gitlab.com/pycqa/flake8/merge_requests/448
.. _GitLab!451:
 https://gitlab.com/pycqa/flake8/merge_requests/451

Links

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