[3.10] gh-143935: Email preserve parens when folding comments (GH-143936) (#144350)

Fix a bug in the folding of comments when flattening an email message
using a modern email policy. Comments consisting of a very long sequence of
non-foldable characters could trigger a forced line wrap that omitted the
required leading space on the continuation line, causing the remainder of
the comment to be interpreted as a new header field. This enabled header
injection with carefully crafted inputs.
(cherry picked from commit 17d1490)

Co-authored-by: Seth Michael Larson seth@python.org
Co-authored-by: Denis Ledoux dle@odoo.com

- Issue: Fix folding of long comments of unfoldable characters in email headers #143935

Signed-off-by: Edgar Ramírez Mondragón <edgarrm358@gmail.com>
Co-authored-by: Seth Michael Larson <seth@python.org>