py3: use email.parser module to parse email messages
Before this patch we use email.Parser.Parser() from the email module which is
not available on Python 3.
On Python 2:
>>> import email
>>> import email.parser as emailparser
>>> email.Parser.Parser is emailparser.Parser
True
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}