Update djoser to 1.3.1

[pyup-bot]

This PR updates djoser from 1.1.5 to 1.3.1.

Changelog

1.3.1

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

* Fix issue with circular import

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

1.3.0

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

* Split user serializers (thanks to joshua-s)
* Add Django 2.1 to tox.ini
* Update travis.yml

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

1.2.0

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

* Refactor urls to use new RESTful ViewSets
* Retain old urls for compatibility
* Add Django 2.0 to tox.ini
* Add DRF 3.8 to tox.ini
* Drop Django 1.10 support
* Update requirements.txt
* Update travis.yml
* Update .gitignore

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

Links

• PyPI: https://pypi.org/project/djoser
• Changelog: https://pyup.io/changelogs/djoser/
• Repo: https://github.com/sunscrapers/djoser