Switch test dependency to mox3

mox is pretty much unmaintained these days, however the OpenStack
project are actively maintaining mox3 (a Python 3 compatibile fork
with some other improvements).

websockify seems quite happy to use mox3 instead, so switch the
test dependency and associated imports to use mox3.

tests/test_websocket.py

@@ -22,7 +22,7 @@ import select
 import shutil
 import socket
 import ssl
-import stubout
+from mox3 import stubout
 import sys
 import tempfile
 import unittest