general: allow slicing querysets (#43328)

wcs/variables.py

@@ -139,7 +139,8 @@ class LazyFormDefObjectsManager(object):
 
     def __getitem__(self, key):
         try:
-            int(key)
+            if not isinstance(key, slice):
+                int(key)
         except ValueError:
             # A django template doing formdef.objects.drafts would start by
             # doing ['drafts'], that would raise TypeError and then continue