for loop script over telnet in Python 3.5 is not working

[bowlofred]

tn.write(b"int loopback " + str(n) + "\n")

The expression you have in the argument is a bytestring, a str, and a str, all added together. But you can't add bytestrings and strs.

>>> b"foo" + "foo"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can't concat str to bytes

If whatever you need to feed bytes to needs UTF-8, then you should generally encode your str to turn it into bytes.

>>> b"foo" + "foo".encode()
b'foofoo'