Update flake8 to 3.7.7
Update flake8 to 3.7.7


This PR updates [flake8](https://pypi.org/project/flake8) from **3.7.6** to **3.7.7**.



<details>
  <summary>Changelog</summary>
  
  
   ### 3.7.7
   ```
   -------------------

You can view the `3.7.7 milestone`_ on GitLab for more details.

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

- Fix crahes in plugins causing ``flake8`` to hang while unpickling errors (See
  also `GitLab!308`_, `GitLab505`_)


.. all links
.. _3.7.7 milestone:
    https://gitlab.com/pycqa/flake8/milestones/30

.. issue links
.. _GitLab505:
    https://gitlab.com/pycqa/flake8/issues/505

.. merge request links
.. _GitLab!308:
    https://gitlab.com/pycqa/flake8/merge_requests/308
   ```
   
  
</details>


 

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


mozilla-services/tecken,mozilla-services/tecken,mozilla-services/tecken,mozilla-services/tecken