[3.14] gh-132617: Fix `dict.update()` mutation check (gh-134815) (gh-135581)

Use `ma_used` instead of `ma_keys->dk_nentries` for modification check
so that we only check if the dictionary is modified, not if new keys are
added to a different dictionary that shared the same keys object.
(cherry picked from commit d8994b0a77cc9821772d05db00a6ab23382fa17d)

Co-authored-by: Sam Gross <colesbury@gmail.com>