bpo-33012: Fix invalid function cast warnings with gcc 8 for METH_NOARGS. (GH-6030)

METH_NOARGS functions need only a single argument but they are cast
into a PyCFunction, which takes two arguments.  This triggers an
invalid function cast warning in gcc8 due to the argument mismatch.
Fix this by adding a dummy unused argument.
diff --git a/Modules/_io/fileio.c b/Modules/_io/fileio.c
index 0d5bf3b..c0e43e0 100644
--- a/Modules/_io/fileio.c
+++ b/Modules/_io/fileio.c
@@ -1123,7 +1123,7 @@
 }
 
 static PyObject *
-fileio_getstate(fileio *self)
+fileio_getstate(fileio *self, PyObject *Py_UNUSED(ignored))
 {
     PyErr_Format(PyExc_TypeError,
                  "cannot serialize '%s' object", Py_TYPE(self)->tp_name);