Update django to 3.0.8

[pyup-bot]

This PR updates Django from 3.0.7 to 3.0.8.

Changelog

3.0.8

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

*July 1, 2020*

Django 3.0.8 fixes several bugs in 3.0.7.

Bugfixes
========

* Fixed messages of ``InvalidCacheKey`` exceptions and ``CacheKeyWarning``
warnings raised by cache key validation (:ticket:`31654`).

* Fixed a regression in Django 3.0.7 that caused a queryset crash when grouping
by a many-to-one relationship (:ticket:`31660`).

* Reallowed, following a regression in Django 3.0, non-expressions having a
``filterable`` attribute to be used as the right-hand side in queryset
filters (:ticket:`31664`).

* Fixed a regression in Django 3.0.2 that caused a migration crash on
PostgreSQL when adding a foreign key to a model with a namespaced
``db_table`` (:ticket:`31735`).

* Added compatibility for ``cx_Oracle`` 8 (:ticket:`31751`).


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

Links

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

[coveralls]

Coverage remained the same at 100.0% when pulling 6cac5f4 on pyup-update-django-3.0.7-to-3.0.8 into 717fcdb on master.