code: def _next_page(self):
          
          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
docs: 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.