add Sinhala locale

[yichidev]

Pull Request Checklist

Thank you for taking the time to improve Arrow! Before submitting your pull request, please check all appropriate boxes:

• [x] 🧪 Added tests for changed code.
• [x] 🛠️ All tests pass when run locally (run tox or make test to find out!).
• [x] 🧹 All linting checks pass when run locally (run tox -e lint or make lint to find out!).
• [ ] 📚 Updated documentation for changed code.
• [ ] ⏩ Code is up-to-date with the master branch.

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Description of Changes

Add Sinhala locale. Sinhala awares time frame format and describe function, takes into account the differences between general, past, and future forms (three different suffixes). So we do not copy the original timeframes dictionary as timeframes_only_distance, we wrote a new timeframes_only_distance dictionary :)

[codecov[bot]]

Codecov Report

Merging #1043 (747da78) into master (9b45719) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master     #1043   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         2168      2208   +40     
  Branches       345       348    +3     
=========================================
+ Hits          2168      2208   +40     

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

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