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
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state