Fix import on python 2

Author: patrick91

graphene_django/rest_framework/serializer_converter.py

@@ -1,10 +1,12 @@
-from functools import singledispatch
-
 from django.core.exceptions import ImproperlyConfigured
 from rest_framework import serializers
 
 import graphene
 
+from ..utils import import_single_dispatch
+
+singledispatch = import_single_dispatch()
+
 
 @singledispatch
 def convert_serializer_field(field):
@@ -53,5 +55,3 @@ def convert_serializer_field_to_bool(field):
 @required_if_input_and_required
 def convert_serializer_field_to_float(field):
     return graphene.Float
-
-