fix test_py3kwarns

The fact that this was failing and went unnoticed so long seems like a good argument for being able to enable and disble py3kwarnings through Python.
diff --git a/Lib/plat-mac/terminalcommand.py b/Lib/plat-mac/terminalcommand.py
index 238781a..ef8cb6c 100644
--- a/Lib/plat-mac/terminalcommand.py
+++ b/Lib/plat-mac/terminalcommand.py
@@ -15,7 +15,7 @@
 #
 
 from warnings import warnpy3k
-warnpy3k("In 3.x, the terminalcommand module is removed.")
+warnpy3k("In 3.x, the terminalcommand module is removed.", stacklevel=2)
 
 import time
 import os