      }
  
      def __init__(self, **kwargs):
          self.pk_field = kwargs.pop('pk_field', None)
          super(PrimaryKeyRelatedField, self).__init__(**kwargs)
  
      def use_pk_only_optimization(self):
          return True
  
      def to_internal_value(self, data):
 
</s> remove         super(NullBooleanField, self).__init__(**kwargs)
 </s> add         super().__init__(**kwargs) </s> remove         super(_UnvalidatedField, self).__init__(*args, **kwargs)
 </s> add         super().__init__(*args, **kwargs) </s> remove         super(IPAddressField, self).__init__(**kwargs)
 </s> add         super().__init__(**kwargs) </s> remove         super(StringRelatedField, self).__init__(**kwargs)
 </s> add         super().__init__(**kwargs) </s> remove         super(SlugRelatedField, self).__init__(**kwargs)
 </s> add         super().__init__(**kwargs)
        super().__init__(**kwargs)
      }
  
      def __init__(self, **kwargs):
          self.pk_field = kwargs.pop('pk_field', None)
        super().__init__(**kwargs)
  
      def use_pk_only_optimization(self):
          return True
  
      def to_internal_value(self, data):
keep keep keep keep replace keep keep keep keep keep
 Dropped Python 2 compatibility. (#6615)

Thanks to Jon Dufresne (@jdufresne) for review.

Co-authored-by: Asif Saif Uddin <auvipy@gmail.com>
Co-authored-by: Rizwan Mansuri <Rizwan@webbyfox.com> 
rest_framework/relations.py