Update django to 2.2.6

[pyup-bot]

This PR updates django from 2.2.5 to 2.2.6.

Changelog

2.2.6

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

*October 1, 2019*

Django 2.2.6 fixes several bugs in 2.2.5.

Bugfixes
========

* Fixed migrations crash on SQLite when altering a model containing partial
indexes (:ticket:`30754`).

* Fixed a regression in Django 2.2.4 that caused a crash when filtering with a
``Subquery()`` annotation of a queryset containing
:class:`~django.contrib.postgres.fields.JSONField` or
:class:`~django.contrib.postgres.fields.HStoreField` (:ticket:`30769`).


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

Links

• PyPI: https://pypi.org/project/django
• Changelog: https://pyup.io/changelogs/django/
• Homepage: https://www.djangoproject.com/

[codecov]

Codecov Report

Merging #150 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #150   +/-   ##
=======================================
  Coverage   98.85%   98.85%           
=======================================
  Files          35       35           
  Lines         522      522           
=======================================
  Hits          516      516           
  Misses          6        6

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[pyup-bot]

Closing this in favor of #163