cleanup overflowing handling in unicode_decode_call_errorhandler and unicode_encode_ucs1 (closes #22518)

I accidentally committed this in 6375bf34fff6; this changeset addresses the
last review comments on the patch.

This is a perennial question and something someone opens a ticket for probably
every other month or so, so I'm surprised we didn't already have a FAQ entry
for it.

The original patch was written by M. Votz, refined first by Ezio Melotti and
further refined by me.

Patch by Yongzhi Pan.

Pointed out by Arfrever.

Patch by Ian Cordasco, approved by Éric Araujo.

Original patch (and the idea of using _structure) by Abhilash Raj.

Patch by Jonathan Hosmer.

Note that the email version string in 3.3 and 3.4 is inaccurate, since it
still exists but wasn't updated while the code was.

Oleg Oshmyan.

Patch by Anastasia Filatova.

Patch by Joshua Moore-Oliva.

Also fixed the phrasing in a comment.

used in the with statement in generator.

Patch by Terry Chia.

vestigial Makefile step.

directory attributes.

to AbstractTestZip64InSmallFiles.

records if allowZip64 is false.