py3: use email.generator module instead of email.Generator
On py2:
>>> import email
>>> import email.generator as emailgen
>>> email.Generator.Generator is emailgen.Generator
True
email.Generator is not present on Python 3.
Differential Revision: https://phab.mercurial-scm.org/D2454
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}