Update flake8 to 3.9.1

[pyup-bot]

This PR updates flake8 from 3.9.0 to 3.9.1.

Changelog

3.9.1

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

You can view the `3.9.1 milestone`_ on GitHub for more details.

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

- Fix codes being ignored by plugins utilizing ``extend_default_ignore`` (See
also :pull:`1317`)


.. all links
.. _3.9.1 milestone:
 https://github.com/PyCQA/flake8/milestone/38

Links

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