patchbomb: use email.encoders instead of email.Encoders
No idea when this became a thing, but it exists for me in both 2.7 and 3.6.
Differential Revision: https://phab.mercurial-scm.org/D4222
mail: properly handle email addresses typically being unicodes
Visible progress in test-patchbomb.t on Python 3.
Differential Revision: https://phab.mercurial-scm.org/D4221