Update flake8 to 3.9.0

[pyup-bot]

This PR updates flake8 from 3.8.4 to 3.9.0.

Changelog

3.9.0

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

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

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

- Pyflakes has been updated to >= 2.3.0, < 2.4.0 (See also `GitLab!466`_)

- pycodestyle has been updated to >= 2.7.0, < 2.8.0 (See also `GitLab!467`_)

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

- Drop support for python 3.4 (See also `GitLab!457`_)

Features
~~~~~~~~

- Add ``--no-show-source`` option to disable ``--show-source`` (See also
`GitLab!441`_)

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

- Fix handling of ``crlf`` line endings when linting stdin (See also
`GitLab!461`_)


.. all links
.. _3.9.0 milestone:
 https://gitlab.com/pycqa/flake8/-/milestones/38

.. merge request links
.. _GitLab!441:
https://gitlab.com/pycqa/flake8/-/merge_requests/441
.. _GitLab!457:
https://gitlab.com/pycqa/flake8/-/merge_requests/457
.. _GitLab!461:
https://gitlab.com/pycqa/flake8/-/merge_requests/461
.. _GitLab!466:
https://gitlab.com/pycqa/flake8/-/merge_requests/466
.. _GitLab!467:
https://gitlab.com/pycqa/flake8/-/merge_requests/467

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 #252