Update flake8 to 3.9.1

[pyup-bot]

This PR updates flake8 from 3.8.4 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

3.9.0

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

You can view the `3.9.0 milestone`_ on GitHub for more details.

New Dependency Information
~~~~~~~~~~~~~~~~~~~~~~~~~~

- Pyflakes has been updated to >= 2.3.0, < 2.4.0 (See also :issue:`1006`)

- pycodestyle has been updated to >= 2.7.0, < 2.8.0 (See also :issue:`1007`)

Deprecations
~~~~~~~~~~~~

- Drop support for python 3.4 (See also :issue:`1283`)

Features
~~~~~~~~

- Add ``--no-show-source`` option to disable ``--show-source`` (See also
:issue:`995`)

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

- Fix handling of ``crlf`` line endings when linting stdin (See also
:issue:`1002`)


.. all links
.. _3.9.0 milestone:
 https://github.com/pycqa/flake8/milestone/37

Links

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

[pyup-bot]

Closing this in favor of #394