[3.12] pythonGH-89727: Partially fix shutil.rmtree() recursion erro…

…r on deep trees (pythonGH-119634)

Make `shutil._rmtree_unsafe()` call `os.walk()`, which is implemented
without recursion.

`shutil._rmtree_safe_fd()` is not affected and can still raise a recursion
error.

Co-authored-by: Jelle Zijlstra <[email protected]>.
(cherry picked from commit a150679)

Co-authored-by: Barney Gale <[email protected]>