Added note that the atexit module was added for Python 1.6.

Doc/lib/libatexit.tex

@@ -6,6 +6,8 @@
 \sectionauthor{Skip Montanaro}{skip@mojam.com}
 \modulesynopsis{Register and execute cleanup functions.}
 
+\versionadded{1.6}
+
 The \module{atexit} module defines a single function to register
 cleanup functions.  Functions thus registered are automatically
 executed upon normal interpreter termination.