Add early support for Django 3.1.
Review Request #11096 — Created July 23, 2020 and submitted
Django 3.1 is currently in RC, so it will be out before long. To
prepare, we're adding support for 3.1.
There isn't much in the way of changes here, just a new
Model._metathat had to be set for our mock models, removal of
FieldDoesNotExistcompatibility import (this moved from
django.core.exceptionsin Django 1.8),
and some small changes to our tests to handle some minor SQL schema
changes (just bringing back a missing space before
gone since Django 2.0).
Unit tests pass for all supported versions of Django on all databases.