Update CI to use latest versions of pyarrow and numpy. Drop pyarrow 4 test config.

[selitvin]

Does not seem to be anything special in pyarrow 4, so dropped it.

[codecov]

Codecov Report

Base: 86.30% // Head: 86.03% // Decreases project coverage by -0.26% ⚠️

Coverage data is based on head (ee8cbb0) compared to base (4e80f2b).
Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #778      +/-   ##
==========================================
- Coverage   86.30%   86.03%   -0.27%     
==========================================
  Files          85       84       -1     
  Lines        5095     4990     -105     
  Branches      786      785       -1     
==========================================
- Hits         4397     4293     -104     
+ Misses        559      558       -1     
  Partials      139      139              

Impacted Files	Coverage Δ
petastorm/etl/__init__.py	66.66% <0.00%> (-9.53%)	⬇️
petastorm/etl/rowgroup_indexers.py	42.18% <0.00%> (-4.96%)	⬇️
...s/imagenet/tests/test_generate_imagenet_dataset.py	86.84% <0.00%> (-0.97%)	⬇️
petastorm/workers_pool/worker_base.py	90.00% <0.00%> (-0.91%)	⬇️
petastorm/codecs.py	78.74% <0.00%> (-0.81%)	⬇️
petastorm/reader_impl/shuffling_buffer.py	93.65% <0.00%> (-0.72%)	⬇️
petastorm/predicates.py	75.82% <0.00%> (-0.52%)	⬇️
petastorm/ngram.py	87.23% <0.00%> (-0.40%)	⬇️
examples/mnist/tests/test_pytorch_mnist.py	91.11% <0.00%> (-0.38%)	⬇️
petastorm/workers_pool/ventilator.py	92.95% <0.00%> (-0.38%)	⬇️
... and 22 more

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