Update relations.py

Currently if you define the slug field as a nested relationship in a `SlugRelatedField` while many=False, it will cause an attribute error. For example:

For this code:
```
class SomeSerializer(serializers.ModelSerializer):
    some_field= serializers.SlugRelatedField(queryset=SomeClass.objects.all(), slug_field="foo__bar")
```
The POST request (or save operation) should work just fine, but if you use GET, then it will fail with Attribute error:

> AttributeError: 'SomeClass' object has no attribute 'foo__bar'

Thus I am handling nested relation here. Reference: https://stackoverflow.com/questions/75878103/drf-attributeerror-when-trying-to-creating-a-instance-with-slugrelatedfield-and/75882424#75882424

rest_framework/relations.py

@@ -2,6 +2,7 @@ import contextlib
 import sys
 from collections import OrderedDict
 from urllib import parse
+from operator import attrgetter
 
 from django.core.exceptions import ImproperlyConfigured, ObjectDoesNotExist
 from django.db.models import Manager
@@ -464,7 +465,11 @@ class SlugRelatedField(RelatedField):
             self.fail('invalid')
 
     def to_representation(self, obj):
-        return getattr(obj, self.slug_field)
+        slug = self.slug_field
+        if "__" in slug:
+            # handling nested relationship defined by double underscore
+            slug = slug.replace('__', '.')
+        return attrgetter(obj, slug)
 
 
 class ManyRelatedField(Field):