Add Serbian locale

[cyriaka90]

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.
• [x] ⏩ Code is up-to-date with the master branch.

Description of Changes

Adds Serbian locale. Serbian actually can be written with both Latin or Cyrillic script - I chose the Latin one here. Not sure if arrow could support both?

[codecov[bot]]

Codecov Report

Merging #980 (020ae47) into master (3606a80) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #980   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         2056      2074   +18     
  Branches       330       332    +2     
=========================================
+ Hits          2056      2074   +18     

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

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[krisfremen]

@cyriaka90 hvala lepo!

we have an ongoing issue to discuss how to implement native alphabets vs latin alphabet in #974

[cyriaka90]

we have an ongoing issue to discuss how to implement native alphabets vs latin alphabet in #974

Thanks for the hint!

[anishnya]

Merged in. Keep up the great work @cyriaka90!

[cyriaka90]

Thank you! :D