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 };