bpo-37207: enable vectorcall for type.__call__ (GH-14588)

Base PR for other PRs that want to play with `type.__call__` such as #13930 and #14589. The author is really @markshannon I just made the PR. https://bugs.python.org/issue37207 Automerge-Triggered-By: @encukou

Author: jdemeyer

Misc/NEWS.d/next/C API/2019-07-07-10-37-07.bpo-37207.SlVNky.rst

@@ -0,0 +1,3 @@
+The vectorcall protocol is now enabled for ``type`` objects: set
+``tp_vectorcall`` to a vectorcall function to be used instead of ``tp_new``
+and ``tp_init`` when calling the class itself.

Objects/typeobject.c

@@ -3614,7 +3614,7 @@ PyTypeObject PyType_Type = {
  sizeof(PyHeapTypeObject), /* tp_basicsize */
  sizeof(PyMemberDef), /* tp_itemsize */
  (destructor)type_dealloc, /* tp_dealloc */
- 0,  /* tp_vectorcall_offset */
+ offsetof(PyTypeObject, tp_vectorcall), /* tp_vectorcall_offset */
  0, /* tp_getattr */
  0, /* tp_setattr */
  0, /* tp_as_async */
@@ -3629,7 +3629,8 @@ PyTypeObject PyType_Type = {
  (setattrofunc)type_setattro, /* tp_setattro */
  0, /* tp_as_buffer */
  Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
- Py_TPFLAGS_BASETYPE | Py_TPFLAGS_TYPE_SUBCLASS, /* tp_flags */
+ Py_TPFLAGS_BASETYPE | Py_TPFLAGS_TYPE_SUBCLASS |
+ _Py_TPFLAGS_HAVE_VECTORCALL, /* tp_flags */
  type_doc, /* tp_doc */
  (traverseproc)type_traverse, /* tp_traverse */
  (inquiry)type_clear, /* tp_clear */