Use \versionchanged to indicate when the second parameter to utime()

was allowed to be None.

Doc/lib/libos.tex

@@ -706,8 +706,9 @@ Availability: Macintosh, \UNIX{}, Windows.
 Set the access and modified times of the file specified by \var{path}.
 If \var{times} is \code{None}, then the file's access and modified
 times are set to the current time.  Otherwise, \var{times} must be a
-2-tuple of numbers, of the form \var{(atime, mtime)} which is used to
-set the access and modified times, respectively.
+2-tuple of numbers, of the form \code{(\var{atime}, \var{mtime})}
+which is used to set the access and modified times, respectively.
+\versionchanged[added support for \code{None} for \var{times}]{1.6}
 Availability: Macintosh, \UNIX{}, Windows.
 \end{funcdesc}