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

ListSeparator should not be encoded. This could happen when a long line
pushes its separator to the next line, which would have been encoded.
(cherry picked from commit 09fab93c3d857496c0bd162797fab816c311ee48)

Co-authored-by: Thomas Weißschuh <thomas@t-8ch.de>