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/base/models/tax.py

@@ -118,6 +118,9 @@ class TaxRule(LoggedModel):
     )
     custom_rules = models.TextField(blank=True, null=True)
 
+    class Meta:
+        ordering = ('event', 'rate', 'id')
+
     def allow_delete(self):
         from pretix.base.models.orders import OrderFee, OrderPosition