Update gunicorn to 20.0.4

[pyup-bot]

This PR updates gunicorn from 19.8.1 to 20.0.4.

Changelog

19.9.0

===================

- fix: address a regression that prevented syslog support from working
(:issue:`1668`, :pr:`1773`)
- fix: correctly set `REMOTE_ADDR` on versions of Python 3 affected by
`Python Issue 30205 <https://bugs.python.org/issue30205>`_
(:issue:`1755`, :pr:`1796`)
- fix: show zero response length correctly in access log (:pr:`1787`)
- fix: prevent raising :exc:`AttributeError` when ``--reload`` is not passed
in case of a :exc:`SyntaxError` raised from the WSGI application.
(:issue:`1805`, :pr:`1806`)
- The internal module ``gunicorn.workers.async`` was renamed to ``gunicorn.workers.base_async``
since ``async`` is now a reserved word in Python 3.7.
(:pr:`1527`)

Links

• PyPI: https://pypi.org/project/gunicorn
• Changelog: https://pyup.io/changelogs/gunicorn/
• Homepage: http://gunicorn.org

[pyup-bot]

Closing this in favor of #62