[3.10] gh-99889: Fix directory traversal security flaw in uu.decode() (GH-104096) (#104330)

gh-99889: Fix directory traversal security flaw in uu.decode() (GH-104096)

* Fix directory traversal security flaw in uu.decode()
* also check absolute paths and os.altsep
* Add a regression test.

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(cherry picked from commit 0aeda297931820436a50b78f4f7f0597274b5df4)


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Co-authored-by: Sam Carroll <70000253+samcarroll42@users.noreply.github.com>