Add attribution for patch co-author.

Doc/whatsnew/3.1.rst

@@ -164,7 +164,7 @@ Some smaller changes made to the core Python language are:
     ...         if '<critical>' in line:
     ...             outfile.write(line)
 
-  (Contributed by Georg Brandl;
+  (Contributed by Georg Brandl and Mattias Brändström;
   `appspot issue 53094 <http://codereview.appspot.com/53094>`_.)
 
 * ``round(x, n)`` now returns an integer if *x* is an integer.