…ythonGH-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 0aeda29)

Co-authored-by: Sam Carroll <70000253+samcarroll42@users.noreply.github.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org> [Google]