Update South Korea holidays, add l10n support

[PPsyrius]

Proposed change

• Add South Korea holidays (en_US, kr, th localization).
• Implement Election Date Calculation (as per Article 34 of the Public Official Election Act ).
• Inclusion of all Temporary Public Holidays from 1948-2023 as part of the StaticHolidays class.
• Revise holiday logic work from the 1st-18th amendment of the Public Holidays Act.

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 added references to all holidays information sources used in this PR
• The code style looks good: make pre-commit command generates no changes
• All tests pass locally: make test, make tox (we strongly encourage adding tests to your code)

[PPsyrius]

This PR is marked as a draft to wait for #1535 's eventual merge (mostly so that we can include the historic, short observances in 1959-1960, 1989 done (as seen here; Point 4.2.1 Before the 2010s ).

[coveralls]

Pull Request Test Coverage Report for Build 6793343944

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• 75 of 75 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 99.978%

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Totals
Change from base Build 6775600278:	0.0%
Covered Lines:	10200
Relevant Lines:	10200

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💛 - Coveralls

[KJhellico]

@PPsyrius, it's just great!

[PPsyrius]

Turns out #1535's new special_holidays_observed requires the usage of ObservedHolidayBase

We may as well go and revamp the existing codes for _add_alt_holiday( ) and _add_three_day_holiday( ) at this rate, as the change to the new observed class requires at least one observed_rule.

[KJhellico]

I'm trying to migrate South Korea to ObservedHolidays, but I'm not satisfied with the result yet. Their rules are quite complex and unique.

[arkid15r]

Smooth migration plus l10n, 2 in 1!

A few suggestions:

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
4 Code Smells

No Coverage information
No Duplication information

[arkid15r]

🇰🇷 LGTM