Extend HolidayBase::categories to accept a single value

[arkid15r]

Proposed change

Extend HolidayBase::category to accept a single value

• Introduce helpers::_normalize_arguments handler
• Update tests
• Update examples documentation

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)

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

No Coverage information
0.8% Duplication

[coveralls]

Pull Request Test Coverage Report for Build 6828746460

• 10 of 10 (100.0%) changed or added relevant lines in 2 files 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 6828617897:	0.0%
Covered Lines:	10240
Relevant Lines:	10240

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💛 - Coveralls

[KJhellico]

It's great! Args like categories=(BANK,) didn't look good.