Add reference implementation for PEP 443

PEP accepted: http://mail.python.org/pipermail/python-dev/2013-June/126734.html
diff --git a/Modules/Setup.dist b/Modules/Setup.dist
index 8ea2b5e..d4e1fe1 100644
--- a/Modules/Setup.dist
+++ b/Modules/Setup.dist
@@ -116,6 +116,7 @@
 _operator _operator.c	        # operator.add() and similar goodies
 _collections _collectionsmodule.c # Container types
 itertools itertoolsmodule.c    # Functions creating iterators for efficient looping 
+atexit atexitmodule.c      # Register functions to be run at interpreter-shutdown
 
 # access to ISO C locale support
 _locale _localemodule.c  # -lintl
@@ -170,7 +171,6 @@
 #_weakref _weakref.c	# basic weak reference support
 #_testcapi _testcapimodule.c    # Python C API test module
 #_random _randommodule.c	# Random number generator
-#atexit atexitmodule.c      # Register functions to be run at interpreter-shutdown
 #_elementtree -I$(srcdir)/Modules/expat -DHAVE_EXPAT_CONFIG_H -DUSE_PYEXPAT_CAPI _elementtree.c	# elementtree accelerator
 #_pickle _pickle.c	# pickle accelerator
 #_datetime _datetimemodule.c	# datetime accelerator