router: fix routing with serial lines

bug: self.fno instead of service.fno

router.py

@@ -281,7 +281,7 @@ class SerialHandler(Router):
         self.fno = self.serial.fileno()
         super().__init__(client, service)
         service.readers[client.fileno()] = self.request
-        service.readers[service.fno] = self.reply
+        service.readers[self.fno] = self.reply
 
     def read(self):
         return self.serial.read(self.serial.in_waiting)