Document when unittest was added to Python.

Doc/lib/libunittest.tex

@@ -7,6 +7,7 @@
 \sectionauthor{Steve Purcell}{stephen\textunderscore{}purcell@yahoo.com}
 \sectionauthor{Fred L. Drake, Jr.}{fdrake@acm.org}
 
+\versionadded{2.1}
 
 The Python unit testing framework, often referred to as ``PyUnit,'' is
 a Python language version of JUnit, by Kent Beck and Erich Gamma.