Prefer io.BytesIO over six; available on all supported Pythons (urlli…

…b3#1435)

On all supported Pythons, the io.BytesIO is always a stream
implementation using an in-memory bytes buffer. Makes code slightly more
forward compatible by reducing use of the six module.

src/urllib3/contrib/appengine.py

@@ -39,6 +39,7 @@
 """
 
 from __future__ import absolute_import
+import io
 import logging
 import os
 import warnings
@@ -53,7 +54,6 @@
     SSLError
 )
 
-from ..packages.six import BytesIO
 from ..request import RequestMethods
 from ..response import HTTPResponse
 from ..util.timeout import Timeout
@@ -239,15 +239,15 @@ def _urlfetch_response_to_http_response(self, urlfetch_resp, **response_kw):
         original_response = HTTPResponse(
             # In order for decoding to work, we must present the content as
             # a file-like object.
-            body=BytesIO(urlfetch_resp.content),
+            body=io.BytesIO(urlfetch_resp.content),
             msg=urlfetch_resp.header_msg,
             headers=urlfetch_resp.headers,
             status=urlfetch_resp.status_code,
             **response_kw
         )
 
         return HTTPResponse(
-            body=BytesIO(urlfetch_resp.content),
+            body=io.BytesIO(urlfetch_resp.content),
             headers=urlfetch_resp.headers,
             status=urlfetch_resp.status_code,
             original_response=original_response,