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.

Python/_warnings.c

@@ -476,7 +476,7 @@ warn_explicit(PyObject *category, PyObject *message,
     }
     else {
         text = message;
-        message = PyObject_CallFunction(category, "O", message);
+        message = _PyObject_CallArg1(category, message);
         if (message == NULL)
             goto cleanup;
     }