Update django to 2.1.3

[pyup-bot]

This PR updates Django from 2.1.2 to 2.1.3.

Changelog

2.1.3

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

*November 1, 2018*

Django 2.1.3 fixes several bugs in 2.1.2.

Bugfixes
========

* Fixed a regression in Django 2.0 where combining ``Q`` objects with ``__in``
lookups and lists crashed (:ticket:`29838`).

* Fixed a regression in Django 1.11 where ``django-admin shell`` may hang
on startup (:ticket:`29774`).

* Fixed a regression in Django 2.0 where test databases aren't reused with
``manage.py test --keepdb`` on MySQL (:ticket:`29827`).

* Fixed a regression where cached foreign keys that use ``to_field`` were
incorrectly cleared in ``Model.save()`` (:ticket:`29896`).

* Fixed a regression in Django 2.0 where ``FileSystemStorage`` crashes with
``FileExistsError`` if concurrent saves try to create the same directory
(:ticket:`29890`).


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

Links

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