mail: import email.utils not email.Utils
email.Utils was renamed to email.utils in Python 2.5. But it appears
at some point before Python 2.7 (possibly 2.5) email.Utils was aliased
to the new home. So email.utils should be safe for all versions of
Python we support.
Differential Revision: https://phab.mercurial-scm.org/D2168