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/_tkinter.c

@@ -2768,7 +2768,7 @@ TimerHandler(ClientData clientData)
 
     ENTER_PYTHON
 
-    res = _PyObject_CallNoArg(func);
+    res = PyObject_CallNoArgs(func);
     Py_DECREF(func);
     Py_DECREF(v); /* See Tktt_New() */