SortableStackedInlineBase fails if the fields is defined as a tuple

[peterfarrell]

SortableStackedInlineBase fails if the fields is defined as a tuple in an admin.py instead of a list (both tuples and lists are acceptable in admin.py). Suit failed on trying to use remove() on an immutable tuple (because it doesn't exist in tuples).

PR puts in an assert to make sure we are not dealing with a tuple and at least give direction on how to fix.

File "/usr/local/lib/python2.7/dist-packages/django/contrib/admin/options.py" in get_formset
  1,600.             fields = flatten_fieldsets(self.get_fieldsets(request, obj))
File "/usr/local/lib/python2.7/dist-packages/suit/admin.py" in get_fieldsets
  97.                     fields.remove(self.sortable)

Exception Type: AttributeError at /admin/myapp/someobject/2/
Exception Value: 'tuple' object has no attribute 'remove'

[darklow]

Refactored a bit. Thanks for PR.

[peterfarrell]

Thanks!