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ARROW-16806: [CI][Python] Bump required setuptools version (#13361)

Our `setup.py` script now uses the `find_namespace_packages` function which is not provided by old setuptools versions.

Fixes source release verification script on Ubuntu 18.04.

Authored-by: Raúl Cumplido <raulcumplido@gmail.com>
Signed-off-by: Antoine Pitrou <pitrou@free.fr>