Add Latvian 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.

If you have any questions about your code changes or any of the points above, please submit your questions along with the pull request and we will try our best to help!

Description of Changes

Adds another locale: Latvian. :)

[codecov[bot]]

Codecov Report

Merging #951 (78e15e7) into master (85964d0) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #951   +/-   ##
=======================================
  Coverage   99.74%   99.74%           
=======================================
  Files          10       10           
  Lines        1927     1937   +10     
  Branches      313      313           
=======================================
+ Hits         1922     1932   +10     
  Misses          4        4           
  Partials        1        1           

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

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[cyriaka90]

You're welcome! Thanks for merging!