According to the docs this method should return a str, but it returns bytes
https://docs.python.org/3.6/library/xdrlib.html#xdrlib.Unpacker.unpack_string

I can see three options:
 a default encoding should be applied
 the documentation updated to make clear this returns bytes
 the method removed and just rely on the existing unpack_bytes which does what you'd expect (and in fact the two methods are aliases)