Release 5.0.0

Changes:

• Removed compat shims for obsolete Django versions
• Switch from deprecated django.conf.urls.url to django.urls.path
• Use query .exists() instead of .count() > 0
• Added testing for Django 3.0
• Drop support for Python 2
• Django generic JSONField support, details documented here
• Django 3.2+ compatibility
• Use _default_manager instead of objects