[3.12] gh-126862: Use `Py_ssize_t` instead of `int` when processing the number of super-classes (GH-127523) (#128700)

gh-126862: Use `Py_ssize_t` instead of `int` when processing the number of super-classes  (GH-127523)
(cherry picked from commit 2fcdc8488c32d18f4567f797094068a994777f16)

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>