Class first_arg_kw_only

tests/test_kwargs_and_defaults.cpp:306–306

    // https://github.com/pybind/pybind11/pull/3402#issuecomment-963341987

    struct first_arg_kw_only {};
    py::class_<first_arg_kw_only>(m, "first_arg_kw_only")
        .def(py::init([](int) { return first_arg_kw_only(); }),
             py::kw_only(), // This being before any args was broken