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ARROW-3160: [Python] Improve pathlib.Path support in parquet and filesystem modules

- organize parquet test files to be consistent with the recently added orc [subdir](https://github.com/apache/arrow/tree/master/python/pyarrow/tests/data)
- use pathlib paths in parquet and orc tests

I intend to consolidate parquet test suite a bit in followup PRs.

Author: Krisztián Szűcs <szucs.krisztian@gmail.com>

Closes #2506 from kszucs/ARROW-3160 and squashes the following commits:

bf1fe1f8 <Krisztián Szűcs> use _has_pathlib
8b023034 <Krisztián Szűcs> compatibility with py35
dc306003 <Krisztián Szűcs> reorganizte utility functions in parquet and filasystem modules; support pathlib.Path in Filesystem API
062ef56a <Krisztián Szűcs> global tempdir fixture returns pathlib object
7c4d72c7 <Krisztián Szűcs> fallback to pathlib2 for older python
c550e1ba <Krisztián Szűcs> update parquet and orc tests to use pathlib
ee494170 <Krisztián Szűcs> use pathlib in orc and parquet tests; move parquet test files to its own directory