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/auth.py

@@ -111,6 +111,7 @@ class User(AbstractBaseUser, PermissionsMixin, LoggingMixin):
     class Meta:
         verbose_name = _("User")
         verbose_name_plural = _("Users")
+        ordering = ('email',)
 
     def save(self, *args, **kwargs):
         self.email = self.email.lower()

src/pretix/base/models/fields.py

@@ -37,7 +37,7 @@ class MultiStringField(TextField):
     def get_prep_lookup(self, lookup_type, value):  # NOQA
         raise TypeError('Lookups on multi strings are currently not supported.')
 
-    def from_db_value(self, value, expression, connection, context):
+    def from_db_value(self, value, expression, connection):
         if value:
             return [v for v in value.split(DELIMITER) if v]
         else:

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