[3.12] GH-89727: Fix FD leak on `os.fwalk()` generator finalization. (GH-119766) (#119768)

GH-89727: Fix FD leak on `os.fwalk()` generator finalization. (GH-119766)

Follow-up to 3c890b50. Ensure we `os.close()` open file descriptors when
the `os.fwalk()` generator is finalized.
(cherry picked from commit a5fef800d31648d19cecc240a2fa0dc71371753e)

Co-authored-by: Barney Gale <barney.gale@gmail.com>