Use explicit `many=True` in PaginationSerializer on `object_serializer` instantiation

Since implicit serialization of many objects without `many=True` is no longer valid (AFAIR it was depraceted in 2.3.*) `PaginationSerializer`  should use this flag to serialize each of it's pages.

Without this fix page serialization will fall if these conditions are met:
* pagination serializer has custom `object_serializer_class` Meta opt
* custom `object_serializer_class` has any fields
* `list` (or any other iterable with __len__ atribute) was passed to Paginator.

In result it is then impossible to serialize objects that are not models with pagination.
This print statement is going to need to be dropped.