fix: use the filter method of the superclass, if it exists

Python does not pass the object from one parent classes' method to the
next, we have to call super() ourselves, if the method exists.

Closes: #263

apis_core/core/mixins.py

@@ -40,6 +40,8 @@ class ListViewObjectFilterMixin:
     """
 
     def filter_queryset(self, queryset):
+        if hasattr(super(), 'filter_queryset'):
+            queryset = super().filter_queryset(queryset)
         if hasattr(settings, "APIS_LIST_VIEW_OBJECT_FILTER"):
             return settings.APIS_LIST_VIEW_OBJECT_FILTER(self, queryset)
         return queryset