add Luxembourgish locale

[yichidev]

Pull Request Checklist

• [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.

Description of Changes

Add Luxembourgish locale :) Here is the reference.

Note: hook id: black is failed when running test locally at first place :(

[codecov[bot]]

Codecov Report

Merging #993 (5830d55) into master (7c9632c) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #993   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         2083      2111   +28     
  Branches       332       333    +1     
=========================================
+ Hits          2083      2111   +28     

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

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