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.

Doc/c-api/object.rst

@@ -253,6 +253,16 @@ Object Protocol
    and ``0`` otherwise.  This function always succeeds.
 
 
+.. c:function:: PyObject* PyObject_CallNoArgs(PyObject *callable)
+
+   Call a callable Python object *callable* without any arguments.
+
+   Returns the result of the call on success, or raise an exception and return
+   *NULL* on failure.
+
+   .. versionadded:: 3.9
+
+
 .. c:function:: PyObject* PyObject_Call(PyObject *callable, PyObject *args, PyObject *kwargs)
 
    Call a callable Python object *callable*, with arguments given by the