py3: don't index into bytes in phabricator's _tokenize()
`phabread`ing a stack using `hg phabread :D1234` under py3 will currently die
with a KeyError because it will index into `b':D1234'` and return `58` instead
of `b':'` as a token.
Differential Revision: https://phab.mercurial-scm.org/D7129
(binary:application/octet-stream)