Merge substituted holidays with special holidays

[KJhellico]

Proposed change

Merge substituted holidays with special holidays (into single special_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

[coveralls]

Pull Request Test Coverage Report for Build 7064413923

• 26 of 26 (100.0%) changed or added relevant lines in 7 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 7053108539:	0.0%
Covered Lines:	10287
Relevant Lines:	10287

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💛 - Coveralls

[arkid15r]

Looks better with less code (as we discussed).

Some concerns to address:

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
1 Code Smell

No Coverage information
0.0% Duplication

[arkid15r]

LGTM