[3.12] gh-117313: Fix re-folding email messages containing non-standard line separato...

[3.12] gh-117313: Fix re-folding email messages containing non-standard line separators (GH-117369) (GH-117971)

Only treat '\n', '\r' and '\r\n' as line separators in re-folding the email
messages.  Preserve control characters '\v', '\f', '\x1c', '\x1d' and '\x1e'
and Unicode line separators '\x85', '\u2028' and '\u2029' as is.
(cherry picked from commit aec1dac4efe36a7db51f08385ddcce978814dbe3)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>