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/_elementtree.c

@@ -3892,7 +3892,7 @@ _elementtree_XMLParser_close_impl(XMLParserObject *self)
     }
     else if (self->handle_close) {
         Py_DECREF(res);
-        return _PyObject_CallNoArg(self->handle_close);
+        return PyObject_CallNoArgs(self->handle_close);
     }
     else {
         return res;