Fixed #6223 -- When determining if terminal supports color, don't call...

Fixed #6223 -- When determining if terminal supports color, don't call `isatty` if it doesn't exist, thanks mamadou.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7202 bcc190cf-cafb-0310-a4f2-bffc1f526a37