bpo-38076: Make struct module PEP-384 compatible (#15805)

* PEP-384 _struct

* More PEP-384 fixes for _struct

Summary: Add a couple of more fixes for `_struct` that were previously missed such as removing `tp_*` accessors and using `PyBytesWriter` instead of calling `PyBytes_FromStringAndSize` with `NULL`. Also added a test to confirm that `iter_unpack` type is still uninstantiable.

* 📜🤖 Added by blurb_it.