Bug of the Day: :issue:`19957`
==============================

So, now that we've solidified what values
:class:`~email.message.Message`._payload can hold, we can do some
simplifications.  Thanks to Vajrasky Kok for pointing out, in :issue:`19957`, a
simplification in :func:`~email.encoders.encode_7or8bit`, and to Serhiy
Storchaka for pointing out an additional simplification in that same function.

The elimination of the special case for ISO-2022 is especially welcome,
as special cases are notoriously fragile.  I tried to eliminate it
once before, but at that time the model was still a bit inconsistent,
so there were cases where ISO-2022 would fail to be encoded correctly.
With the recent change to set_payload, this should no longer be a
problem.