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ARROW-2859: [Python] Accept buffer-like objects as sources in open_file, open_stream APIs

The behavior had been to treat a string-like object like a file name; we didn't have any APIs that made use of this fact, and I think that being able to read a stream from an object importing the buffer protocol is much more convenient and natural as `pa.open_stream(buf)` than `pa.open_stream(pa.BufferReader(buf))`.

I may look at quickly adding support for pathlib.Path objects here.

I also added the precursor for addressing ARROW-2807

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

Closes #2314 from wesm/ARROW-2859 and squashes the following commits:

b64a828c <Wes McKinney> Fix docstrings
5cc363f8 <Wes McKinney> Amend usages of get_result, add FutureWarning
b11e5328 <Wes McKinney> Add pathlib test. Refactor to use pytest
53f32e84 <Wes McKinney> Add test for stream from buffer protocol
a6fc8f1c <Wes McKinney> Do not try to open file from buffer input, add use_memory_map flag