Import httplib from six to support Python 3

In Python 3, httplib moved to http.client.
Use six in order to import the correct library on both Python 2 and
Python 3.

Change-Id: I6f43faf453809eed3f2e88fd587e941c80204793
Closes-Bug: #1623447

os_api_ref/http_codes.py

@@ -13,7 +13,7 @@
 from docutils import nodes
 from docutils.parsers.rst.directives.tables import Table
 from docutils.statemachine import ViewList
-from httplib import responses
+from six.moves.http_client import responses
 import yaml
 
 # cache for file -> yaml so we only do the load and check of a yaml

requirements.txt

@@ -4,5 +4,6 @@
 
 pbr>=1.6 # Apache-2.0
 PyYAML>=3.10.0 # MIT
+six>=1.9.0 # MIT
 sphinx!=1.3b1,<1.4,>=1.2.1 # BSD
 openstackdocstheme>=1.5.0 # Apache-2.0