Update six to a version with raises_from
diff --git a/requirements.txt b/requirements.txt
index 2ec028d..5e2d881 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -3,4 +3,4 @@
 # but setuptools can't cope with conflicts in setup_requires, so thats
 # unversioned.
 pbr>=0.11
-six>=1.7
+six>=1.9