py3: use r'' for group name arguments to MatchObjects in phabricator.py

Authored by Kwan.

Description

py3: use r'' for group name arguments to MatchObjects in phabricator.py

MatchObject group names are strings, not bytes. Using bytes in py3 leads to an
IndexError.

Differential Revision: https://phab.mercurial-scm.org/D6111