Add Mauritania holidays

[kasya]

Proposed change

Add Mauritania holidays.

Type of change

• New country/market holidays support (thank you!)
• Supported country/market holidays update (calendar discrepancy fix, localization)
• Existing code/documentation/test/process quality improvement (best practice, cleanup, refactoring, optimization)
• Dependency update (version deprecation/upgrade)
• Bugfix (non-breaking change which fixes an issue)
• Breaking change (a code change causing existing functionality to break)
• New feature (new python-holidays functionality in general)

Checklist

• I've followed the contributing guidelines
• I've run make pre-commit, it didn't generate any changes
• I've run make test, all tests passed locally

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (fa3feb8) to head (5494437).

Additional details and impacted files

@@            Coverage Diff            @@
##               dev     #1884   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          184       185    +1     
  Lines        11194     11222   +28     
  Branches      1765      1766    +1     
=========================================
+ Hits         11194     11222   +28     

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[KJhellico]

@kasya, it's great work! There hasn't been new countries for a long time. Please take a look at few suggestions.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[arkid15r]

LGTM,
thank you @kasya

[KJhellico]

LGTM.