Define __all__ for models.py within each app

netbox/virtualization/models.py

@@ -11,6 +11,14 @@ from utilities.models import ChangeLoggedModel
 from .choices import *
 
 
+__all__ = (
+    'Cluster',
+    'ClusterGroup',
+    'ClusterType',
+    'VirtualMachine',
+)
+
+
 #
 # Cluster types
 #

netbox/virtualization/tests/test_models.py

@@ -1,3 +1,4 @@
+from django.core.exceptions import ValidationError
 from django.test import TestCase
 
 from virtualization.models import *