Use six.moves to handle a py2+py3 import

Signed-off-by: Anthony Sottile <asottile@umich.edu>

docker/transport/unixconn.py

 import six
 import requests.adapters
 import socket
+from six.moves import http_client as httplib
 
 from .. import constants
 
-if six.PY3:
-    import http.client as httplib
-else:
-    import httplib
-
 try:
     import requests.packages.urllib3 as urllib3
 except ImportError: