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ARROW-4413: [Python] Fix pa.hdfs.connect() on Python 2

I can't actually test this works on a HDFS filesystem, but this should fix the logical error.

Author: Antoine Pitrou <antoine@python.org>

Closes #3975 from pitrou/ARROW-4413-hdfs-py2 and squashes the following commits:

4cb4a8b07 <Antoine Pitrou> ARROW-4413:  Fix pa.hdfs.connect() on Python 2