bpo-37194: Add a new public PyObject_CallNoArgs() function (GH-13890)

Add a new public PyObject_CallNoArgs() function to the C API: call a
callable Python object without any arguments.

It is the most efficient way to call a callback without any argument.
On x86-64, for example, PyObject_CallFunctionObjArgs(func, NULL)
allocates 960 bytes on the stack per call, whereas
PyObject_CallNoArgs(func) only allocates 624 bytes per call.

It is excluded from stable ABI 3.8.

Replace private _PyObject_CallNoArg() with public
PyObject_CallNoArgs() in C extensions: _asyncio, _datetime,
_elementtree, _pickle, _tkinter and readline.

Modules/readline.c

@@ -867,7 +867,7 @@ on_hook(PyObject *func)
     int result = 0;
     if (func != NULL) {
         PyObject *r;
-        r = _PyObject_CallNoArg(func);
+        r = PyObject_CallNoArgs(func);
         if (r == NULL)
             goto error;
         if (r == Py_None)