First, the documentation says that syntax for 'O!' conversion is
object(type='name_of_Python_type'), but actually the type argument should be the name of C structure which represents Python type. I.e. object(type='list') is rejected, and object(type='PyList_Type') is accepted.

Second, from this declaration

/*[clinic]
module spam
spam.ham

    items: object(type='PyList_Type')
    /

[clinic]*/

Argument Clinic generates following parsing code:

    if (!PyArg_ParseTuple(args,
        "O!:ham",
        PyList_Type, &items))
        goto exit;

It is wrong, should be &PyList_Type.