Fix SdBusSlot_close having too few arguments

The second argument must be present even if it is not used to be
able to cast the function to `PyCFunction`.

src/sdbus/sd_bus_internals.c

@@ -54,7 +54,7 @@ static void SdBusSlot_dealloc(SdBusSlotObject* self) {
         SD_BUS_DEALLOC_TAIL;
 }
 
-static PyObject* SdBusSlot_close(SdBusSlotObject* self) {
+static PyObject* SdBusSlot_close(SdBusSlotObject* self, PyObject* Py_UNUSED(args)) {
         sd_bus_slot_unref(self->slot_ref);
         self->slot_ref = NULL;