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ARROW-2660: [Python] Experimental zero-copy pickling

Zero-copy pickling of buffers and buffer-based objects will be possible using PEP 574 (if/when accepted).  The PyPI backport "pickle5" helps us test that possibility.

Author: Antoine Pitrou <antoine@python.org>

Closes #2161 from pitrou/ARROW-2660-zero-copy-pickling and squashes the following commits:

50f0491 <Antoine Pitrou> Fix test on Python 2.7 (hopefully)
132939c <Antoine Pitrou> Add pickle5 to CI environments
892302a <Antoine Pitrou> ARROW-2660:  Zero-copy pickling