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.

Objects/call.c

@@ -70,6 +70,14 @@ _Py_CheckFunctionResult(PyObject *callable, PyObject *result, const char *where)
 
 /* --- Core PyObject call functions ------------------------------- */
 
+/* Call a callable Python object without any arguments */
+PyObject *
+PyObject_CallNoArgs(PyObject *func)
+{
+    return _PyObject_CallNoArg(func);
+}
+
+
 PyObject *
 _PyObject_FastCallDict(PyObject *callable, PyObject *const *args,
                        size_t nargsf, PyObject *kwargs)