  
      def __init__(self, **kwargs):
          assert 'allow_null' not in kwargs, '`allow_null` is not a valid option.'
          kwargs['allow_null'] = True
          super(NullBooleanField, self).__init__(**kwargs)
  
      def to_internal_value(self, data):
          try:
              if data in self.TRUE_VALUES:
                  return True
 
</s> remove         super(PrimaryKeyRelatedField, self).__init__(**kwargs)
 </s> add         super().__init__(**kwargs) </s> remove         super(SlugRelatedField, 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             field.run_validation(u'slug-99-\u0420')
 </s> add             field.run_validation('slug-99-\u0420')
        super().__init__(**kwargs)
  
      def __init__(self, **kwargs):
          assert 'allow_null' not in kwargs, '`allow_null` is not a valid option.'
          kwargs['allow_null'] = True
        super().__init__(**kwargs)
  
      def to_internal_value(self, data):
          try:
              if data in self.TRUE_VALUES:
                  return True
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/fields.py