Lib/encodings/idna.py claims to do the following when `input` is a
string object (lines 183-184, and see comment line 178: "IDNA allows
decoding to operate on Unicode strings, too"):

            # Force to bytes
            input = bytes(input)

This is obviously wrong, lacking an encoding parameter. It doesn't seem
to be covered in the test suite, and I don't know what the proper
semantics should be, so I leave it to someone else to find a fix.