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/pretix/plugins/ticketoutputpdf/models.py

@@ -73,3 +73,4 @@ class TicketLayoutItem(models.Model):
 
     class Meta:
         unique_together = (('item', 'layout', 'sales_channel'),)
+        ordering = ("id",)