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 concrete_model

key in Model._meta that had to be set for our mock models, removal of

the legacy FieldDoesNotExist compatibility import (this moved from

django.db.models.fields to django.core.exceptions in Django 1.8),

and some small changes to our tests to handle some minor SQL schema

changes (just bringing back a missing space before DESC that's been

gone since Django 2.0).

Unit tests pass for all supported versions of Django on all databases.