Adapt to flake8-import-order

https://review.openstack.org/#/c/432906/
flake8-import-order adds 3 new flake8 warnings:
I100: Your import statements are in the wrong order.
I101: The names in your from import are in the wrong order.
I201: Missing newline between sections or imports.

Signed-off-by: Ben Pfaff <blp@ovn.org>

python/ovs/stream.py

@@ -17,12 +17,12 @@ import os
 import socket
 import sys
 
-import six
-
 import ovs.poller
 import ovs.socket_util
 import ovs.vlog
 
+import six
+
 try:
     from OpenSSL import SSL
 except ImportError: