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ARROW-5596: [Python] Fix Python-3 syntax only in test_flight.py

Even though Flight is only available in Python 3, having Py3-only syntax is enough to break py.test

Author: Wes McKinney <wesm+git@apache.org>

Closes #4553 from wesm/ARROW-5596 and squashes the following commits:

7130d401e <Wes McKinney> Use pathlib backport
7f85e6104 <Wes McKinney> Fix Python-3 syntax only in test_flight.py