Pass Field instead of str to DeferredAttributeTracker constructor

This makes it pass mypy inspection, but there is no test coverage,
so I'm not certain this solution is correct. However, as the code
was already broken, it's unlikely to make things worse.

#559

model_utils/tracker.py

@@ -332,12 +332,12 @@ def _get_field_name(self) -> str:
 
         self.instance._deferred_fields = self.instance.get_deferred_fields()
         for field in self.instance._deferred_fields:
-            field_obj = self.instance.__class__.__dict__.get(field)
+            field_obj: models.Field = self.instance.__class__.__dict__[field]
             if isinstance(field_obj, FileDescriptor):
                 file_descriptor_tracker = FileDescriptorTracker(field_obj.field)
                 setattr(self.instance.__class__, field, file_descriptor_tracker)
             else:
-                deferred_attribute_tracker = DeferredAttributeTracker(field)
+                deferred_attribute_tracker = DeferredAttributeTracker(field_obj)
                 setattr(self.instance.__class__, field, deferred_attribute_tracker)