Add note that difflib was added in Python 2.1.

Doc/lib/libdifflib.tex

@@ -7,6 +7,9 @@
 \sectionauthor{Tim Peters}{tim.one@home.com}
 % LaTeXification by Fred L. Drake, Jr. <fdrake@acm.org>.
 
+\versionadded{2.1}
+
+
 \begin{funcdesc}{get_close_matches}{word, possibilities\optional{,
                  n\optional{, cutoff}}}
   Return a list of the best ``good enough'' matches.  \var{word} is a