The following code causes an assertion failure:
    import _struct
    struct_obj = _struct.Struct.__new__(_struct.Struct)
    struct_obj.iter_unpack(b'foo')

This is because Struct_iter_unpack() (in Modules/_struct.c) assumes that
Struct.__init__() was called, and so it does `assert(self->s_codes != NULL);`.

The same happens in (almost) every method of Struct, and in s_get_format(), so
in all them, too, we would get an assertion failure in case of an uninitialized
Struct object.
The exception is __sizeof__(), which doesn't have an `assert`, and simply
crashes while trying to iterate over `self->s_codes`.


I would open a PR to fix this soon.