With issue 3672 resolved, it is now unnecessary to introduce
an utf-8b codec, since the utf-8 codec will properly report errors
for all byte sequences invalid in UTF-8, including lone surrogates.
Therefore, utf-8b can be implemented solely through the error handler.
Glenn Linderman suggested that the name "python-escape" is not very
descriptive, so I've changed the name to "utf8b".
I've updated the PEP accordingly.