FieldFile subclass with the only aim to remove the instance from the state.
The change introduced in Django 3.1 on FieldFile subclasses results in pickling the
whole instance for every field tracked.
As this is done on the initialization of objects, a simple queryset evaluation on
Django 3.1+ can make the app unusable, as CPU and memory usage gets easily
multiplied by magnitudes.
…model_utils/tracker.py