[1.1.X] Fixed #12048 - MultiWidget does not define `__deepcopy__`

  
Thanks to powderflask for report, test case and initial patch.

Backport of [12739] from trunk


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12740 bcc190cf-cafb-0310-a4f2-bffc1f526a37