D6044: phabricator: convert conduit response JSON unicode to bytes inside callconduit

[Yuya Nishihara]

>   > Perhaps some of `r''` would have to be changed to `b''` since dict keys
>   >  are now byte strings. See the wiki page for our Py3 hacks.
>   > 
>   > https://www.mercurial-scm.org/wiki/Python3
>   
>   Ah, good point, thanks.  Would it be worth keeping the keys as r'' strings?  rapply can fairly easily be extended with an optional notkeys boolean that would allow doing so.

I prefer converting everything to bytes so we won't have to handle both
unicode and byte strings.