Added a version annotation for StringTypes.

Doc/lib/libtypes.tex

@@ -165,4 +165,5 @@ portable than using a sequence of the two string types constructed
 elsewhere since it only contains \code{UnicodeType} if it has been
 built in the running version of Python.  For example:
 \code{isinstance(s, types.StringTypes)}.
+\versionadded{2.2}
 \end{datadesc}