[3.15] gh-154308: Clear file flags in os_helper.rmtree() (GH-154310) (GH-154314)

[3.15] gh-154308: Clear file flags in os_helper.rmtree() (GH-154310) (GH-154314)

On BSD systems a test may leave behind files or directories with flags
such as UF_IMMUTABLE or UF_NOUNLINK set, which prevent the directory from
being modified or removed.  Clear the flags before removing, so cleaning
up a temporary directory does not fail.
(cherry picked from commit 1f649fecb645d70b9c48268839e4177d8fe4e1d1)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>