Fix UnsetType type annotation (#916)

[CharString]

This is consistent with the definition in _core.c:

static int unset_bool(PyObject *obj) {
    return 0;
};

static PyNumberMethods unset_as_number = {
    .nb_bool = unset_bool,
};

PyTypeObject Unset_Type = {
    PyVarObject_HEAD_INIT(NULL, 0)
    .tp_name = "msgspec.UnsetType",
    .tp_doc = unset__doc__,
    .tp_repr = unset_repr,
    .tp_flags = Py_TPFLAGS_DEFAULT,
    .tp_methods = unset_methods,
    .tp_as_number = &unset_as_number,
    .tp_new = unset_new,
    .tp_dealloc = 0,
    .tp_itemsize = 0,
    .tp_basicsize = 0
};