Use type flag instead of custom constructor in `_struct.unpack_iterator`

[chgnrdv]

Currently _struct.unpack_iterator type defines its own constructor just to report to user that it isn't instantiable (with a typo, by the way):

cpython/Modules/_struct.c

Lines 1835 to 1838 in 8d95012

	PyObject *unpackiter_new(PyTypeObject *type, PyObject *args, PyObject *kwds) {
	PyErr_Format(PyExc_TypeError, "Cannot create '%.200s objects", _PyType_Name(type));
	return NULL;
	}

It can be easily removed and replaced by appropriate type flag. I'll submit a PR.

Linked PRs

• gh-104276: Make _struct.unpack_iterator type use type flag instead of custom constructor #104277