email: address list folding encodes list-separator comma

[t-8ch]

Bug report

When during address list folding a separating comma ends up on a folded line containing that is to be unicode-encoded than the separator itself is also unicode encoded. Instead the comma should still a plain comma.

This is rejected by some mail servers:

ZZZ: host YYY said:
 550 Invalid address in message header. Consult RFC2822. (in reply to end of
 DATA command)

Minimal example:

>>> import email
>>> m = email.message.EmailMessage()
>>> t = '01234567890123456789012345678901234567890123456789012345678901@example.org, ä <foo@example.org>'
>>> m['to'] = t
>>> m.as_string()
'to: 01234567890123456789012345678901234567890123456789012345678901@example.org\n =?utf-8?q?=2C?= =?utf-8?q?=C3=A4?= <foo@example.org>\n\n'

Your environment

• CPython versions tested on: 3.10.9, latest main (e47b139)
• Operating system and architecture: ArchLinux x86_64

Linked PRs

• GH-100884: email/_header_value_parser: don't encode list separators #100885
• [3.12] gh-100884: email/_header_value_parser: don't encode list separators (GH-100885) #115592
• [3.11] gh-100884: email/_header_value_parser: don't encode list separators (GH-100885) #115593