bpo-33029: Fix signatures of getter and setter functions. (GH-10746)

Fix also return type for few other functions (clear, releasebuffer).

Modules/mmapmodule.c

@@ -656,7 +656,7 @@ mmap_move_method(mmap_object *self, PyObject *args)
 }
 
 static PyObject *
-mmap_closed_get(mmap_object *self)
+mmap_closed_get(mmap_object *self, void *Py_UNUSED(ignored))
 {
 #ifdef MS_WINDOWS
     return PyBool_FromLong(self->map_handle == NULL ? 1 : 0);