[3.12] gh-127541: Update os.walk example (GH-127765) (#131873)

gh-127541: Update os.walk example (GH-127765)

Update os.walk example to be more modern, skip `__pycache__` dirs rather than `CVS` dirs.
(cherry picked from commit a5949986d631391d37b1b329ad8badcf2000f9a9)

Co-authored-by: Totosuki <116938397+totosuki@users.noreply.github.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>