update 3.7 whatsnew entry about Unicode version (GH-7480)


Followup to 4705ea38c900f068fd262aca02943896d1123544 and bpo-33778.
(cherry picked from commit 34b734699b19d826f861b604dd77e82beed95f17)

Co-authored-by: Benjamin Peterson <benjamin@python.org>
1 file changed