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.

Lib/test/test_struct.py

@@ -679,6 +679,10 @@ class UnpackIteratorTest(unittest.TestCase):
         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

+The struct module is now PEP-384 compatible
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