Update django-autoslug to 1.9.9

[pyup-bot]

This PR updates django-autoslug from 1.9.3 to 1.9.9.

Changelog

1.9.9

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

* Prevent situation in which slug could end in dash or underscore
* Remove support for end-of-life Python & Django versions

1.9.8

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

Move FieldDoesNotExist import for compatibility with Django 3.1

1.9.7

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

Fix assertion error on empty slug

1.9.6

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

Handle timezones for datetime fields

1.9.5

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

Add license to sdist and wheels

1.9.4

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

New features:

- Add `manager_name` kwarg to enable using custom managers from abstract models
- Add compatibility for Django versions 1.10, 1.11, 2.0, and 2.1
- Transfer project to new maintainer

Links

• PyPI: https://pypi.org/project/django-autoslug
• Changelog: https://pyup.io/changelogs/django-autoslug/
• Repo: https://github.com/justinmayer/django-autoslug/archive/master.zip
• Docs: https://pythonhosted.org/django-autoslug/