bpo-35059: PyObject_INIT() casts to PyObject* (GH-10674)

PyObject_INIT() and PyObject_INIT_VAR() now cast their first argument
to PyObject*, as done in Python 3.7.

Revert partially commit b4435e20a9.

PC/winreg.c

@@ -459,7 +459,7 @@ PyHKEY_FromHKEY(HKEY h)
     op = (PyHKEYObject *) PyObject_MALLOC(sizeof(PyHKEYObject));
     if (op == NULL)
         return PyErr_NoMemory();
-    PyObject_INIT((PyObject *)op, &PyHKEY_Type);
+    PyObject_INIT(op, &PyHKEY_Type);
     op->hkey = h;
     return (PyObject *)op;
 }