I am not sure how the parser work, so I can't comment if your change to
it is correct. It would probably a better idea to keep it for later,
when the semantic differences between str8 and bytes objects will be
resolved.

I think changing b'' to buffer(b''), in the tests, is not a good idea.
These tests will need to be changed again when bytes() will be changed
to str8().

Your changes to PyString and PyBytes look good to me. As I said on the
mailing list, I think the for-loops in bytes_repr() should be changed to:

    while (*quote_prefix)
        *p++ = *quote_prefix++;

I attached updated patches with the above change. I also replaced
bytes() for buffer() in PyBytes's docstrings.