Serhiy's patch looks to me like it would pragmatically cover all the cases most likely to affect porting efforts: using the standard library bytes<->bytes codecs through the convenience methods.

For a Python 2 backport, I'm slightly more concerned with exposing the argument in the constructor signature, but see value in being consistent with Python 3 if anyone decides to use this to check a custom codec.

The main advantage this approach has over the typecheck based approach is that it can correctly warn about "data".encode("hex"), while a typecheck based approach can't distinguish that from "text".encode("ascii")