Added missing import.

rest_framework/serializers.py

@@ -21,6 +21,7 @@ from django.core.paginator import Page
 from django.db import models
 from django.forms import widgets
 from django.utils.datastructures import SortedDict
+from django.core.exceptions import ObjectDoesNotExist
 from rest_framework.compat import get_concrete_model, six
 from rest_framework.settings import api_settings