Update flake8 to 3.7.6

[pyup-bot]

This PR updates flake8 from 3.7.4 to 3.7.6.

Changelog

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

3.7.5

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

You can view the `3.7.5 milestone`_ on GitLab for more details.

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

- Fix reporting of pyflakes "referenced before assignment" error (See also
`GitLab!301`_, `GitLab503`_)


.. all links
.. _3.7.5 milestone:
 https://gitlab.com/pycqa/flake8/milestones/28

.. issue links
.. _GitLab503:
 https://gitlab.com/pycqa/flake8/issues/503

.. merge request links
.. _GitLab!301:
 https://gitlab.com/pycqa/flake8/merge_requests/301

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 #313