py3: port metaedit extension to Python 3
Without these changes, this extension is responsible for a test of
test failures.
Differential Revision: https://phab.mercurial-scm.org/D2150
py3: pass system string to email.message.Message.set_type()
Python 3 insists the type is a str.
Differential Revision: https://phab.mercurial-scm.org/D2149