Use the compat layer for atexit functionality

Author: Joshua Harlow

kazoo/testing/harness.py

@@ -1,10 +1,12 @@
 """Kazoo testing harnesses"""
-import atexit
+
 import logging
 import os
 import uuid
 import unittest
 
+from kazoo import python2atexit as atexit
+
 from kazoo.client import KazooClient
 from kazoo.exceptions import NotEmptyError
 from kazoo.protocol.states import (