Django 3.1rc1 update / bring back sqlite support

I tried to update to django 3.1.rc1 which went without bigger issues. I made two upstream pull requests:

• https://github.com/django-parler/django-parler/pull/272 (no workaround possible)
• https://github.com/model-bakers/model_bakery/pull/85 (workaround in 3ce0ef6d)

The big new feature in 3.1 is a cross-db JSONField. This was the original reason why we switched from the default sqlite to postgres in !305 (merged). So I tried reverting that change. Unfortunately, there are now additional incompatibilities with sqlite:

• !1361 (merged) added timedelta multiplication (see https://code.djangoproject.com/ticket/25287)
  • we could store a timedelta directly instead of a number of minutes
• !1400 (merged) added a dependency on geodjango for which sqlite does not have a backend
  • we could disable geo features in development

As you can see, we could make this work. But I don't think it is worth it at this moment.