JSONfield compatability

This package is currently importing JSONfield from either django-jsonfield + django-jsonfield-compat or django_mysql, however the django-jsonfield/django-jsonfield-compat combination only supports up to django 1.9.

I can make a PR for importing from the current preferred location, django.contrib.postgres.fields, but I just need to know if you’d also want to continue support for django =< 1.9 and therefore allow import from the existing packages.

I am not a mysql user, but it appears no change is needed there, and importing from django_mysql is still the preferred method.

[travijuu]

@cb109 When I try to use django-activity-stream with Django 3.0, I got error below

django.core.exceptions.ImproperlyConfigured: You must either install django-jsonfield + django-jsonfield-compat, or django-mysql as an alternative, if you wish to use a JSONField on your actions

Note: both django-jsonfield and django-jsonfield-compat already installed.

When I dig into error, I see invalid import statement of six in this link.

from django.utils import six

after I replaced this line with import six then Django 3.0 started working.

[lociii]

Switch from all the different libraries to Django built-in JSONField or the backported package has been merged.