This also replaces the PolymorphicInlineModelAdmin.get_get_child_inline_instance()
and BasePolymorphicModelFormSet._construct_form() lookup with UnsupportedChildType
(cherry picked from commit cafaf95f06)
In Python 3.4 trying to generate a polymorphic_inlineformset results in a TypeError with the following message: "'KeysView' object does not support indexing". This solves that problem by ensuring that `child_models` is a list, and thus can be referenced by index.