Add and improve Unicode-related unit tests for e-mail sending.
Review Request #11681 — Created June 27, 2021 and submitted
This adds some unit tests and updates existing unit tests for the e-mail
code to ensure that Unicode content works correctly and consistently
between Python 2 and 3.
There are no fixes needed along with this, though these tests do depend
on fixes for Djblets's e-mail header and address building code.
Unit tests pass on Python 2 and 3.