[3.12] gh-94673: Ensure subtypes are readied only once in math.trunc() (gh-105465) (gh-105471)

Fixes a typo in d2e2e53.
(cherry picked from commit 5394bf92aa6516feb3322d8372d1579bd9c1417b)

Co-authored-by: neonene <53406459+neonene@users.noreply.github.com>