Made LazyPolymorphicTypeQueryset lazier to account for Django 2.1 changes.
Made LazyPolymorphicTypeQueryset lazier to account for Django 2.1 changes.

ModelChoiceField._set_queryset(queryset) calls queryset.all() on Django 2.1+ in
order to clear possible cached results.

Since no results might have been cached before _setup() is called it's safe to
keep deferring until something else is accessed.

charettes/django-polymodels