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

hgext/phabricator.py

@@ -1274,8 +1274,8 b' def _tokenize(text):'
             yield (b'symbol', symbol, pos)
             pos += len(symbol)
         else:  # special char, ignore space
-            if text[pos] != b' ':
-                yield (text[pos], None, pos)
+            if text[pos : pos + 1] != b' ':
+                yield (text[pos : pos + 1], None, pos)
             pos += 1
     yield (b'end', None, pos)