Update flake8 to 3.7.6
Update flake8 to 3.7.6


This PR updates [flake8](https://pypi.org/project/flake8) from **3.7.5** to **3.7.6**.



<details>
  <summary>Changelog</summary>
  
  
   ### 3.7.6
   ```
   -------------------

You can view the `3.7.6 milestone`_ on GitLab for more details.

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

- Fix ``--per-file-ignores`` for multi-letter error codes (See also
  `GitLab!303`_, `GitLab507`_)

- Improve flake8 speed when only 1 filename is passed (See also `GitLab!305`_)


.. all links
.. _3.7.6 milestone:
    https://gitlab.com/pycqa/flake8/milestones/29

.. issue links
.. _GitLab507:
    https://gitlab.com/pycqa/flake8/issues/507

.. merge request links
.. _GitLab!303:
    https://gitlab.com/pycqa/flake8/merge_requests/303
.. _GitLab!305:
    https://gitlab.com/pycqa/flake8/merge_requests/305
   ```
   
  
</details>


 

<details>
  <summary>Links</summary>
  
  - PyPI: https://pypi.org/project/flake8
  - Changelog: https://pyup.io/changelogs/flake8/
  - Repo: https://gitlab.com/pycqa/flake8
</details>


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