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()