Update flake8 to 3.7.3

[pyup-bot]

This PR updates flake8 from 3.7.1 to 3.7.3.

Changelog

3.7.3

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

You can view the `3.7.3 milestone`_ on GitLab for more details.

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

- Fix imports of ``typing`` in python 3.5.0 / 3.5.1 (See also `GitLab!294`_,
`GitLab498`_)

- Fix ``flake8 --statistics`` (See also `GitLab!295`_, `GitLab499`_)

- Gracefully ignore ``flake8-per-file-ignores`` plugin if installed (See also
`GitLab!297`_, `GitLab495`_)

- Improve error message for malformed ``per-file-ignores`` (See also
`GitLab!298`_, `GitLab489`_)


.. all links
.. _3.7.3 milestone:
 https://gitlab.com/pycqa/flake8/milestones/26

.. issue links
.. _GitLab489:
 https://gitlab.com/pycqa/flake8/issues/489
.. _GitLab495:
 https://gitlab.com/pycqa/flake8/issues/495
.. _GitLab498:
 https://gitlab.com/pycqa/flake8/issues/498
.. _GitLab499:
 https://gitlab.com/pycqa/flake8/issues/499

.. merge request links
.. _GitLab!294:
 https://gitlab.com/pycqa/flake8/merge_requests/294
.. _GitLab!295:
 https://gitlab.com/pycqa/flake8/merge_requests/295
.. _GitLab!297:
 https://gitlab.com/pycqa/flake8/merge_requests/297
.. _GitLab!298:
 https://gitlab.com/pycqa/flake8/merge_requests/298

3.7.2

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

You can view the `3.7.2 milestone`_ on GitLab for more details.

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

- Fix broken ``flake8 --diff`` (regressed in 3.7.0) (See also `GitLab!292`_,
`GitLab490`_)

- Fix typo in plugin exception reporting (See also `GitLab!275`_,
`GitLab491`_)

- Fix ``AttributeError`` while attempting to use the legacy api (regressed in
3.7.0) (See also `GitLab!293`_, `GitLab497`_)

.. all links
.. _3.7.2 milestone:
 https://gitlab.com/pycqa/flake8/milestones/25

.. issue links
.. _GitLab490:
 https://gitlab.com/pycqa/flake8/issues/490
.. _GitLab491:
 https://gitlab.com/pycqa/flake8/issues/491
.. _GitLab497:
 https://gitlab.com/pycqa/flake8/issues/497

.. merge request links
.. _GitLab!292:
 https://gitlab.com/pycqa/flake8/merge_requests/292
.. _GitLab!275:
 https://gitlab.com/pycqa/flake8/merge_requests/275
.. _GitLab!293:
 https://gitlab.com/pycqa/flake8/merge_requests/293

Links

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