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/PixMapWrapper.py b/Lib/plat-mac/PixMapWrapper.py
index 45cb5f9..5ce3043 100644
--- a/Lib/plat-mac/PixMapWrapper.py
+++ b/Lib/plat-mac/PixMapWrapper.py
@@ -7,7 +7,7 @@
 
 
 from warnings import warnpy3k
-warnpy3k("In 3.x, the PixMapWrapper module is removed.")
+warnpy3k("In 3.x, the PixMapWrapper module is removed.", stacklevel=2)
 
 from Carbon import Qd
 from Carbon import QuickDraw