#18584: make doctest examples in email documentation pass.

With the exception of the 'as_string' call in the policy docs.
That one is a separate issue.

Note that when building the docs sphinx is complaining about
.. testcleanup:: being an invalid directive.  I don't know
why this is, as I'm following the sphinx docs...but fortunately
the action is to omit the text in the body, so the generated
documentation is correct.