Update flake8 to 3.7.7

[pyup-bot]

This PR updates flake8 from 3.7.6 to 3.7.7.

Changelog

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

Links

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

[codecov]

Codecov Report

Merging #969 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #969   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           9      9           
  Lines         422    422           
  Branches       10     10           
=====================================
  Hits          422    422

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