ARROW-4434: [Python] Allow creating trivial StructArray

[pitrou]

Also fix an unrelated flake8 error with the latest flake8.

[codecov-io]

Codecov Report

Merging #3610 into master will decrease coverage by 21.19%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #3610      +/-   ##
==========================================
- Coverage   87.75%   66.56%   -21.2%     
==========================================
  Files         673      322     -351     
  Lines       82763    47952   -34811     
  Branches     1069        0    -1069     
==========================================
- Hits        72628    31918   -40710     
- Misses      10020    16034    +6014     
+ Partials      115        0     -115

Impacted Files	Coverage Δ
python/pyarrow/filesystem.py	51.32% <0%> (ø)	⬆️
python/pyarrow/tests/test_convert_builtin.py	99.52% <100%> (ø)	⬆️
python/pyarrow/array.pxi	69.19% <92.3%> (+0.21%)	⬆️
cpp/src/arrow/pretty_print.h	0% <0%> (-100%)	⬇️
cpp/src/parquet/murmur3.h	0% <0%> (-100%)	⬇️
cpp/src/arrow/util/memory.h	0% <0%> (-100%)	⬇️
cpp/src/arrow/testing/util.h	0% <0%> (-100%)	⬇️
cpp/src/arrow/compute/kernels/aggregate.cc	0% <0%> (-100%)	⬇️
cpp/src/arrow/io/hdfs-internal.h	0% <0%> (-100%)	⬇️
cpp/src/arrow/util/sse-util.h	0% <0%> (-100%)	⬇️
... and 498 more

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[wesm]

+1