Get the next page in the iterator.

        Returns:
            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:
            Page: The next page in the iterator or :data:`None` if
                there are no pages left.
        """
        if not self._has_next_page():
            return None

        if self.next_page_token is not None:
            setattr(self._request, self._request_token_field, self.next_page_token)

        response = self._method(self._request)

        self.next_page_token = getattr(response, self._response_token_field)
        items = getattr(response, self._items_field)
        page = Page(self, items, self.item_to_value)

        return page