28 tests passes now.

rest_framework/serializers.py

+import six
+
 import copy
 import datetime
 import types
@@ -54,7 +56,7 @@ def _get_declared_fields(bases, attrs):
     Note that all fields from the base classes are used.
     """
     fields = [(field_name, attrs.pop(field_name))
-              for field_name, obj in attrs.items()
+              for field_name, obj in list(six.iteritems(attrs))
               if isinstance(obj, Field)]
     fields.sort(key=lambda x: x[1].creation_counter)
 
@@ -63,7 +65,7 @@ def _get_declared_fields(bases, attrs):
     # in order to the correct order of fields.
     for base in bases[::-1]:
         if hasattr(base, 'base_fields'):
-            fields = base.base_fields.items() + fields
+            fields = list(base.base_fields.items()) + fields
 
     return SortedDict(fields)
 
@@ -315,8 +317,8 @@ class BaseSerializer(Field):
         return self.object
 
 
-class Serializer(BaseSerializer):
-    __metaclass__ = SerializerMetaclass
+class Serializer(six.with_metaclass(SerializerMetaclass, BaseSerializer)):
+    pass
 
 
 class ModelSerializerOptions(SerializerOptions):