whatsnew-3.10: Mention PEP 647 in the Release highlights section. (GH-99853)

Mention PEP 647 in the Release highlights section.

Also re-ordered the list so it matches the order in the details sections below.
(cherry picked from commit d74a58872c243bc766055d42f8280721629e3c82)

Co-authored-by: Yilei "Dolee" Yang <yileiyang@google.com>