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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.03%. Comparing base (4e80f2b) to head (ee8cbb0).
⚠️ Report is 17 commits behind head on master.

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              

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