Refactor of Slavic Locales for Future Dehumanize Support

[anishnya]

Pull Request Checklist

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• 🧪 Added tests for changed code.
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Description of Changes

This is a refactor of the locales that derive from the Slavic Base Locale. These changes add what I believe to be more clarity to these locales implementations. In addition, these changes make these locales conform to a single unified locale standard I am trying to make, so that all locales will support dehumanize.

[codecov]

Codecov Report

Merging #992 (c17213e) into master (95838a8) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #992   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         2150      2150           
  Branches       339       339           
=========================================
  Hits          2150      2150           

Impacted Files	Coverage Δ
arrow/locales.py	100.00% <100.00%> (ø)

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[krisfremen]

Looks good to me.

I don't think there's a need to do the paucal portion we discussed..