[3.12] gh-114211: Update EmailMessage doc about ordered keys (GH-114224) (#114225)

Ordered keys are no longer unlike 'real dict's.
(cherry picked from commit 8cda72037b262772399b2b7fc36dee9340d74fd6)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>

Miss Islington (bot) committed 2y ago

87198f6c2377a52ee7ec779b7f85daa8deef7f02

Parent: f8fc853

Committed by GitHub <noreply@github.com> on 1/18/2024, 2:55:40 AM