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.

Author: DinoV

Lib/test/test_struct.py

@@ -679,6 +679,10 @@ def _check_iterator(it):
         with self.assertRaises(struct.error):
             s.iter_unpack(b"12")
 
+    def test_uninstantiable(self):
+        iter_unpack_type = type(struct.Struct(">ibcp").iter_unpack(b""))
+        self.assertRaises(TypeError, iter_unpack_type)
+
     def test_iterate(self):
         s = struct.Struct('>IB')
         b = bytes(range(1, 16))

Misc/NEWS.d/next/Core and Builtins/2019-09-09-16-36-37.bpo-38076.C5dVBl.rst

@@ -0,0 +1 @@
+The struct module is now PEP-384 compatible