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/Objects/setobject.c b/Objects/setobject.c
index 47db6b2..80101dd 100644
--- a/Objects/setobject.c
+++ b/Objects/setobject.c
@@ -703,7 +703,7 @@
 }
 
 static PyObject *
-set_pop(PySetObject *so)
+set_pop(PySetObject *so, PyObject *Py_UNUSED(ignored))
 {
     /* Make sure the search finger is in bounds */
     Py_ssize_t i = so->finger & so->mask;
@@ -833,7 +833,7 @@
 }
 
 static PyObject *
-setiter_len(setiterobject *si)
+setiter_len(setiterobject *si, PyObject *Py_UNUSED(ignored))
 {
     Py_ssize_t len = 0;
     if (si->si_set != NULL && si->si_used == si->si_set->used)
@@ -846,7 +846,7 @@
 static PyObject *setiter_iternext(setiterobject *si);
 
 static PyObject *
-setiter_reduce(setiterobject *si)
+setiter_reduce(setiterobject *si, PyObject *Py_UNUSED(ignored))
 {
     PyObject *list;
     setiterobject tmp;
@@ -1175,25 +1175,25 @@
 }
 
 static PyObject *
-set_copy(PySetObject *so)
+set_copy(PySetObject *so, PyObject *Py_UNUSED(ignored))
 {
     return make_new_set_basetype(Py_TYPE(so), (PyObject *)so);
 }
 
 static PyObject *
-frozenset_copy(PySetObject *so)
+frozenset_copy(PySetObject *so, PyObject *Py_UNUSED(ignored))
 {
     if (PyFrozenSet_CheckExact(so)) {
         Py_INCREF(so);
         return (PyObject *)so;
     }
-    return set_copy(so);
+    return set_copy(so, NULL);
 }
 
 PyDoc_STRVAR(copy_doc, "Return a shallow copy of a set.");
 
 static PyObject *
-set_clear(PySetObject *so)
+set_clear(PySetObject *so, PyObject *Py_UNUSED(ignored))
 {
     set_clear_internal(so);
     Py_RETURN_NONE;
@@ -1208,7 +1208,7 @@
     PyObject *other;
     Py_ssize_t i;
 
-    result = (PySetObject *)set_copy(so);
+    result = (PySetObject *)set_copy(so, NULL);
     if (result == NULL)
         return NULL;
 
@@ -1237,7 +1237,7 @@
     if (!PyAnySet_Check(so) || !PyAnySet_Check(other))
         Py_RETURN_NOTIMPLEMENTED;
 
-    result = (PySetObject *)set_copy(so);
+    result = (PySetObject *)set_copy(so, NULL);
     if (result == NULL)
         return NULL;
     if ((PyObject *)so == other)
@@ -1270,7 +1270,7 @@
     int rv;
 
     if ((PyObject *)so == other)
-        return set_copy(so);
+        return set_copy(so, NULL);
 
     result = (PySetObject *)make_new_set_basetype(Py_TYPE(so), NULL);
     if (result == NULL)
@@ -1343,7 +1343,7 @@
     PyObject *result = (PyObject *)so;
 
     if (PyTuple_GET_SIZE(args) == 0)
-        return set_copy(so);
+        return set_copy(so, NULL);
 
     Py_INCREF(so);
     for (i=0 ; i<PyTuple_GET_SIZE(args) ; i++) {
@@ -1542,7 +1542,7 @@
 {
     PyObject *result;
 
-    result = set_copy(so);
+    result = set_copy(so, NULL);
     if (result == NULL)
         return NULL;
     if (set_difference_update_internal((PySetObject *) result, other) == 0)
@@ -1562,7 +1562,7 @@
     int rv;
 
     if (PySet_GET_SIZE(so) == 0) {
-        return set_copy(so);
+        return set_copy(so, NULL);
     }
 
     if (PyAnySet_Check(other)) {
@@ -1630,7 +1630,7 @@
     PyObject *result, *other;
 
     if (PyTuple_GET_SIZE(args) == 0)
-        return set_copy(so);
+        return set_copy(so, NULL);
 
     other = PyTuple_GET_ITEM(args, 0);
     result = set_difference(so, other);
@@ -1681,7 +1681,7 @@
     int rv;
 
     if ((PyObject *)so == other)
-        return set_clear(so);
+        return set_clear(so, NULL);
 
     if (PyDict_CheckExact(other)) {
         PyObject *value;
@@ -1976,7 +1976,7 @@
 If the element is not a member, do nothing.");
 
 static PyObject *
-set_reduce(PySetObject *so)
+set_reduce(PySetObject *so, PyObject *Py_UNUSED(ignored))
 {
     PyObject *keys=NULL, *args=NULL, *result=NULL, *dict=NULL;
     _Py_IDENTIFIER(__dict__);
@@ -2002,7 +2002,7 @@
 }
 
 static PyObject *
-set_sizeof(PySetObject *so)
+set_sizeof(PySetObject *so, PyObject *Py_UNUSED(ignored))
 {
     Py_ssize_t res;
 
@@ -2044,7 +2044,7 @@
 /* set object ********************************************************/
 
 #ifdef Py_DEBUG
-static PyObject *test_c_api(PySetObject *so);
+static PyObject *test_c_api(PySetObject *so, PyObject *Py_UNUSED(ignored));
 
 PyDoc_STRVAR(test_c_api_doc, "Exercises C API.  Returns True.\n\
 All is well if assertions don't fail.");
@@ -2379,7 +2379,7 @@
         PyErr_BadInternalCall();
         return NULL;
     }
-    return set_pop((PySetObject *)set);
+    return set_pop((PySetObject *)set, NULL);
 }
 
 int
@@ -2408,7 +2408,7 @@
     } while(0)
 
 static PyObject *
-test_c_api(PySetObject *so)
+test_c_api(PySetObject *so, PyObject *Py_UNUSED(ignored))
 {
     Py_ssize_t count;
     const char *s;