Get the next page in the iterator.

        Returns:
            Optional[Page]: The next page in the iterator or :data:`None` if
                there are no pages left.

def _next_page(self):
        """Get the next page in the iterator.

        Returns:
            Optional[Page]: The next page in the iterator or :data:`None` if
                there are no pages left.
        """
        if self._has_next_page():
            response = self._get_next_page_response()
            items = response.get(self._items_key, ())
            page = Page(self, items, self.item_to_value)
            self._page_start(self, page, response)
            self.next_page_token = response.get(self._next_token)
            return page
        else:
            return None