Update error re-raising to use a more Python 2/3 compatible version.

rest_framework/request.py

@@ -12,11 +12,13 @@ from __future__ import unicode_literals
 from django.conf import settings
 from django.http import QueryDict
 from django.http.multipartparser import parse_header
+from django.utils import six
 from django.utils.datastructures import MultiValueDict
 from rest_framework import HTTP_HEADER_ENCODING
 from rest_framework import exceptions
 from rest_framework.compat import BytesIO
 from rest_framework.settings import api_settings
+import sys
 
 
 def is_form_media_type(media_type):
@@ -226,7 +228,7 @@ class Request(object):
             try:
                 self._authenticate()
             except AttributeError as error:
-                raise RuntimeError("AttributeError: " + error.message)
+                six.reraise(RuntimeError, "AttributeError: " + str(error), sys.exc_info()[2])
         return self._user
 
     @user.setter