Deal with breaking tests for 3.1

- Django's PostgreSQL JSONField wraps values in a JsonAdapter, so deal
  with that when it happens.  This goes away in Django 3.1.
- Setting related *_id fields clears the actual relation field, so
  trying to fake objects for tests is a problem
- Instance.objects.me() was inappropriately creating stub objects
  every time while running tests, but some of our tests now create
  real db objects. Ditch that logic and use a proper fixture where needed.
- awxkit tox.ini was pinned at Python 3.8

awx/main/fields.py

@@ -94,6 +94,8 @@ class JSONBField(upstream_JSONBField):
     def get_db_prep_value(self, value, connection, prepared=False):
         if connection.vendor == 'sqlite':
             # sqlite (which we use for tests) does not support jsonb;
+            if hasattr(value, 'adapted'):
+                value = value.adapted  # FIXME: Django 3.0 uses JsonAdapter, removed in 3.1
             return json.dumps(value, cls=DjangoJSONEncoder)
         return super(JSONBField, self).get_db_prep_value(value, connection, prepared)