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.

def _next_page(self):
        """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.
        """
        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