Replace PyObject_CallFunction() with fastcall

Replace
    PyObject_CallFunction(func, "O", arg)
and
    PyObject_CallFunction(func, "O", arg, NULL)
with
    _PyObject_CallArg1(func, arg)

Replace
    PyObject_CallFunction(func, NULL)
with
    _PyObject_CallNoArg(func)

_PyObject_CallNoArg() and _PyObject_CallArg1() are simpler and don't allocate
memory on the C stack.

Objects/genobject.c

@@ -1341,7 +1341,7 @@ async_gen_init_hooks(PyAsyncGenObject *o)
         PyObject *res;
 
         Py_INCREF(firstiter);
-        res = PyObject_CallFunction(firstiter, "O", o);
+        res = _PyObject_CallArg1(firstiter, o);
         Py_DECREF(firstiter);
         if (res == NULL) {
             return 1;