Function make_key_iterator

include/pybind11/pybind11.h:3454–3463

          typename KeyType = typename detail::iterator_key_access<Iterator>::result_type,
          typename... Extra>
typing::Iterator<KeyType> make_key_iterator(Iterator first, Sentinel last, Extra &&...extra) {
    return detail::make_iterator_impl<detail::iterator_key_access<Iterator>,
                                      Policy,
                                      Iterator,
                                      Sentinel,
                                      KeyType,
                                      Extra...>(std::forward<Iterator>(first),
                                                std::forward<Sentinel>(last),
                                                std::forward<Extra>(extra)...);
}

/// Makes a python iterator over the values (`.second`) of a iterator over pairs from a
/// first and past-the-end InputIterator.