Adding support for len in pybind11 enum -- version 1

#include <pybind11/pybind11.h>

namespace py = pybind11;

enum class Kind { Kind1 = 1, Kind2, Kind3 };


py::object get_pybind11_metaclass()
{
    auto &internals = py::detail::get_internals();
    return py::reinterpret_borrow<py::object>((PyObject*)internals.default_metaclass);
}

py::object get_standard_metaclass()
{
    auto &internals = py::detail::get_internals();
    return py::reinterpret_borrow<py::object>((PyObject *)&PyType_Type);
}

py::object create_enum_metaclass()
{
    auto pybind11_metaclass = get_pybind11_metaclass();
    auto standard_metaclass = get_standard_metaclass();

    py::dict attributes;
    attributes["__len__"] = py::cpp_function(
        [](py::object cls) {
            return cls.attr("_member_count")();
        }
        , py::is_method(py::none())
        );
    return standard_metaclass(std::string("pybind11_ext_enum")
        , py::make_tuple(pybind11_metaclass)
        , attributes);
}

PYBIND11_MODULE(so07, m)
{
    auto enum_metaclass = create_enum_metaclass();
    py::enum_<Kind>(m, "Kind", py::metaclass(enum_metaclass))
        .value("Kind1", Kind::Kind1)
        .value("Kind2", Kind::Kind2)
        .value("Kind3", Kind::Kind3)
        .def_static("_member_count", []() {
            return 3;
        });
}