ARROW-6092: [Python] Fix C++ arrow-python-test on Python 2.7
ARROW-6092: [Python] Fix C++ arrow-python-test on Python 2.7

I am looking forward to January 1, 2020.

Closes #5145 from wesm/ARROW-6092 and squashes the following commits:

354a76adb <Wes McKinney> Fix arrow-python-test on Python 2.7

Authored-by: Wes McKinney <3be70eed0275ec2606f002bff3514252e145d204@apache.org>
Signed-off-by: Wes McKinney <3be70eed0275ec2606f002bff3514252e145d204@apache.org>

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