Update flake8 to 3.4.0

[pyup-bot]

There's a new version of flake8 available.
You are currently using 3.3.0. I have updated it to 3.4.0

These links might come in handy: PyPI | Changelog | Repo

Changelog

3.4.0

---

You can view the 3.4.0 milestone_ on GitLab for more details.

• Refine logic around --select and --ignore when combined with the
  default values for each. (See also GitLab318_)

• Handle spaces as an alternate separate for error codes, e.g.,
  --ignore 'E123 E234'. (See also GitLab329_)

• Filter out empty select and ignore codes, e.g., --ignore E123,,E234.
  (See also GitLab330_)

.. all links
.. _3.4.0 milestone:
https://gitlab.com/pycqa/flake8/milestones/18

.. issue links
.. _GitLab318:
https://gitlab.com/pycqa/flake8/issues/318
.. _GitLab329:
https://gitlab.com/pycqa/flake8/issues/329
.. _GitLab330:
https://gitlab.com/pycqa/flake8/issues/330

.. merge request links

Got merge conflicts? Close this PR and delete the branch. I'll create a new PR for you.

Happy merging! 🤖