[Checkins] [zopefoundation/zope.sendmail] 2e5699: Work with messages as bytes whenever possible

[Viktor Dick]

  Branch: refs/heads/using-bytes-for-messages
  Home:   https://github.com/zopefoundation/zope.sendmail
  Commit: 2e5699481185c141abc52fe5323d3b5a23420602
      https://github.com/zopefoundation/zope.sendmail/commit/2e5699481185c141abc52fe5323d3b5a23420602
  Author: Viktor Dick <vd at perfact.de>
  Date:   2020-07-20 (Mon, 20 Jul 2020)

  Changed paths:
    M src/zope/sendmail/delivery.py
    M src/zope/sendmail/queue.py
    M src/zope/sendmail/tests/test_delivery.py
    M src/zope/sendmail/tests/test_queue.py

  Log Message:
  -----------
  Work with messages as bytes whenever possible

This is based on the discussion in
`https://github.com/zopefoundation/Products.MailHost/issues/30`
and
`https://github.com/zopefoundation/Products.MailHost/pull/32`.

In order to allow messages with 8bit-encoding to be sent using smtplib,
they have to be prepared as bytes. Since any preparation and header
parsing and manipulation is done in `Products.MailHost`, `zope.sendmail`
only needs to store messages internally as bytes and allow them to be
passed as bytes.