Issue when running telnet program using python3

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I have used this program in automating our network device backup config and

     import getpass 
     import telnetlib 
     import time
     user = input("Enter your telnet username: ") 
     password = getpass.getpass()
 
     f = open("ipadd.txt")
     for line in f:
           print ("Getting running config from devices " + line)
             HOST = line.strip()
             tn = telnetlib.Telnet(HOST, 23, 5)
         
             tn.read_until(b"Username:")
             tn.write(user.encode("ascii")+ b"\n")
             if password:
                 tn.read_until(b"Password:")
                 tn.write(password.encode("ascii")+b"\n") 
         
             tn.read_until(b"#")
             tn.write(b"conf t"+b"\n")
             time.sleep(1)
             tn.write(b"hostname test"+b"\n")
             time.sleep(1)
             tn.write(b"exit"+b"\n")
             time.sleep(1)
             tn.write(b"terminal length 0"+b"\n")
             time.sleep(3)
             tn.write(b"show run"+b"\n")
             time.sleep(3)
             tn.write(b"exit"+b"\n")
             readoutput = tn.read_all().decode('ascii')
             saveoutput = open("device.txt" + HOST, "w")
             saveoutput.write(readoutput)
             saveoutput.close

Error:
**Issue/Error Message:** Im using python3.7 

Traceback (most recent call last):
      File "telnetbu.py", line 27, in <module>
        tn.read_until(b"Username:")
      File "/usr/lib/python3.5/telnetlib.py", line 311, in read_until
        selector.register(self, selectors.EVENT_READ)
      File "/usr/lib/python3.5/selectors.py", line 351, in register
        key = super().register(fileobj, events, data)
      File "/usr/lib/python3.5/selectors.py", line 237, in register
        key = SelectorKey(fileobj, self._fileobj_lookup(fileobj), events, data)
      File "/usr/lib/python3.5/selectors.py", line 224, in _fileobj_lookup
        return _fileobj_to_fd(fileobj)
      File "/usr/lib/python3.5/selectors.py", line 39, in _fileobj_to_fd
        "{!r}".format(fileobj)) from None
    ValueError: Invalid file object: <telnetlib.Telnet object at 0x7fe669487080>

Thank you