[3.10] gh-107902: gh-108948: Don't test setting suid/sgid/sticky on systems that don't support them (GH-108368) (GH-109697) (#125255)

* gh-107902: Don't test setting suid/sgid on systems that don't support them (GH-108368)

* gh-108948: Skip test_tarfile.test_modes() on EFTYPE error (#109697)

On FreeBSD, regular users cannot set the sticky bit. Skip the test if
chmod() fails with EFTYPE error.

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Co-authored-by: Victor Stinner <vstinner@python.org>