ARROW-6403: [Python] Expose FileReader::ReadRowGroups() to Python

[ARF1]

Expose ReadRowGroups to Python to allow efficient filtered reading implementations as suggested @xhochy in #2491 (comment)_

Without this PR users would have to re-implement threaded reads in python.

[ARF1]

As it stands, this PR replicates the C++ interface of parquet in python. Namely it introduces the method read_row_groups(row_groups, ...) in addition to the existing read_row_group(i, ...) in ParquetReader and ParquetFile.

It might be worth considering whether maintainers would prefer avoiding the almost-duplication in the API and instead extend the functionality of the existing read_row_group() to accept a list-like object in addition to the currently accepted integer.

[emkornfield]

@ARF1 Thank you for the PR. Could you open a JIRA and prefix the PR title with: ARROW-{JIRA #}:

https://github.com/apache/arrow/blob/master/.github/CONTRIBUTING.md has more thorough documentation on the process.

[emkornfield]

It also looks like there are flake8 violations:
+python3 -m flake8 --count /home/travis/build/apache/arrow/python
/home/travis/build/apache/arrow/python/pyarrow/parquet.py:193:24: E128 continuation line under-indented for visual indent
/home/travis/build/apache/arrow/python/pyarrow/parquet.py:218:80: E501 line too long (85 > 79 characters)

[ARF1]

@ARF1 Thank you for the PR. Could you open a JIRA and prefix the PR title with: ARROW-{JIRA #}:

https://github.com/apache/arrow/blob/master/.github/CONTRIBUTING.md has more thorough documentation on the process.

@emkornfield Done. Thanks for the guidance. The flake8 violations should now also be fixed. The checks are still running...

[codecov-io]

Codecov Report

Merging #5241 into master will decrease coverage by 22.54%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           master    #5241       +/-   ##
===========================================
- Coverage   87.64%   65.09%   -22.55%     
===========================================
  Files        1033      500      -533     
  Lines      148463    67929    -80534     
  Branches     1437        0     -1437     
===========================================
- Hits       130118    44219    -85899     
- Misses      17983    23710     +5727     
+ Partials      362        0      -362

Impacted Files	Coverage Δ
python/pyarrow/parquet.py	92.34% <100%> (+0.04%)	⬆️
python/pyarrow/_parquet.pyx	91.33% <100%> (+0.04%)	⬆️
python/pyarrow/tests/test_parquet.py	96.61% <100%> (+0.1%)	⬆️
cpp/src/arrow/util/memory.h	0% <0%> (-100%)	⬇️
cpp/src/gandiva/date_utils.h	0% <0%> (-100%)	⬇️
cpp/src/arrow/util/memory.cc	0% <0%> (-100%)	⬇️
cpp/src/gandiva/decimal_type_util.h	0% <0%> (-100%)	⬇️
cpp/src/arrow/filesystem/util_internal.cc	0% <0%> (-100%)	⬇️
cpp/src/arrow/compute/logical_type.h	0% <0%> (-100%)	⬇️
cpp/src/parquet/hasher.h	0% <0%> (-100%)	⬇️
... and 791 more

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[xhochy]

+1, LGTM