Upgrade to Django 2.2 and modern DRF and py.test (#1246)

* Upgrade django and stuff

* Update to Django 2.2 and recent versions of similar packages

* Provide explicit orderings to all models used in paginated queries

* Resolve naive datetime warnings in test suite

* Deal with deprecation warnings

* Fix sqlparse version

src/tests/api/test_items.py

@@ -76,8 +76,8 @@ def cart_position(event, item, variations):
         c = CartPosition.objects.create(
             event=event,
             item=item,
-            datetime=datetime.now(),
-            expires=datetime.now() + timedelta(days=1),
+            datetime=testtime,
+            expires=testtime + timedelta(days=1),
             variation=variations[0],
             price=Decimal("23"),
             cart_id="z3fsn8jyufm5kpk768q69gkbyr5f4h6w"