check for isinstance Page, prevents alt pagination

when using DEFAULT_PAGINATION_SERIALIZER_CLASS and a custom paginator_class
BaseSerializer's checking for isinstance Page doesn't work.

the custom paginator_class has it's own Page that doesn't inherit from
django's base Page and thus its __iter__ method causes many=True, and
iterating over the page and trying to serialize the objects through
DEFAULT_PAGINATION_SERIALIZER_CLASS.

proposed fix is to look for a method that would (likely) only be on a Page type object, rather
than for something to be or inherit from the django pagination Page type.