https://docs.python.org/3/library/sys.html#sys.stdin
All you have to do is use sys.stdin.buffer and sys.stdout.buffer; the caveat is that if sys.stdin has been replaced with a StringIO instance, this won't work. But in Armin's simple cat example, we can trivially make sure that won't happen.
I'd be a lot more willing to listen to this argument if it didn't overlook basic stuff like this.