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

[kszucs]

• organize parquet test files to be consistent with the recently added orc subdir
• use pathlib paths in parquet and orc tests

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

[xhochy]

This PR still has test failures.

[kszucs]

Sorry, forgot to prepend with WIP. I'll request your review when it's done.

[kszucs]

@xhochy What do You think about using pathlib in the implementation too?

[codecov-io]

Codecov Report

Merging #2506 into master will increase coverage by 1.25%.
The diff coverage is 88.65%.

@@            Coverage Diff             @@
##           master    #2506      +/-   ##
==========================================
+ Coverage   86.24%   87.49%   +1.25%     
==========================================
  Files         308      246      -62     
  Lines       47094    43441    -3653     
==========================================
- Hits        40615    38010    -2605     
+ Misses       6405     5431     -974     
+ Partials       74        0      -74

Impacted Files	Coverage Δ
python/pyarrow/io.pxi	60.52% <0%> (-0.41%)	⬇️
python/pyarrow/tests/test_orc.py	100% <100%> (ø)	⬆️
python/pyarrow/parquet.py	93.56% <100%> (+2.22%)	⬆️
python/pyarrow/tests/test_parquet.py	97.41% <100%> (-0.02%)	⬇️
python/pyarrow/filesystem.py	50.27% <50%> (-1.2%)	⬇️
python/pyarrow/util.py	75% <85%> (+16.66%)	⬆️
python/pyarrow/tests/conftest.py	77.04% <91.66%> (+3.46%)	⬆️
rust/src/record_batch.rs
go/arrow/datatype_nested.go
rust/src/array.rs
... and 59 more

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[kszucs]

@xhochy green

[wesm]

+1

[wesm]

I'd be in favor of moving all the test data files to https://github.com/apache/arrow-testing. It would take less stress (for me at least) about adding binary files in the future. There's relatively little need to ship the files (or the unit tests for that matter) with the library