CPython 2.x and Python implementations built directly on it have bytestrings. PyPy does, as far as I know. And Unladen Swallow is a CPython fork, so it does.
But everything else either has strings which are natively Unicode (and hence not byte-based in the sense we want here) or run on platforms where the underlying string abstraction is natively Unicode. This includes Jython, IronPython, Python 3.x, etc.
WSGI would like very much for those platforms to do something else, because it has to expend a bit of verbiage to basically say "shame on you for having these dangerous Unicode strings, don't you dare try to take advantage of them!"
But everything else either has strings which are natively Unicode (and hence not byte-based in the sense we want here) or run on platforms where the underlying string abstraction is natively Unicode. This includes Jython, IronPython, Python 3.x, etc.
WSGI would like very much for those platforms to do something else, because it has to expend a bit of verbiage to basically say "shame on you for having these dangerous Unicode strings, don't you dare try to take advantage of them!"