Fixed #12746 -- Updated sorting calls to use 'key' instead of 'cmp'. This will...

Fixed #12746 -- Updated sorting calls to use 'key' instead of 'cmp'. This will be slightly faster in certain circumstances, but more importantly, is a required step for migration to Python 3. Thanks to Martin van Loewis for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13509 bcc190cf-cafb-0310-a4f2-bffc1f526a37