code: def _next_page(self):
          
          try:
              items = six.next(self._gax_page_iter)
              page = Page(self, items, self.item_to_value)
              self.next_page_token = self._gax_page_iter.page_token or None
              return page
          except StopIteration:
              return None
docs: Get the next page in the iterator.
  
          Wraps the response from the :class:`~google.gax.PageIterator` in a
          :class:`Page` instance and captures some state at each page.
  
          Returns:
              Optional[Page]: The next page in the iterator or :data:`None` if
                    there are no pages left.