Updated openstack/openstack
Project: openstack/stevedore  65fc0d2a364fff791448404acad8a17bd4642a5f

use six.add_metaclass

Using six.add_metaclass instead of "__metaclass__" for Python 3.x
compatibility.

Change-Id: I8bf33fb1770511657587c8f7a33acfcf9a351e79