Keep py3.X compatibility for urllib

Use six.moves.urllib instead of urllib

Partial-Bug: #1280105

Change-Id: Ieace55f7ca0189e40f268a43431ce63130e267be
diff --git a/requirements.txt b/requirements.txt
index e6f46b6..b155078 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -9,3 +9,4 @@
 pkginfo
 PyRSS2Gen
 python-swiftclient
+six>=1.4.1