Add United Kingdom's Whit Monday holidays for pre-1971 years

[PPsyrius]

Proposed change

As spotted in #1664, this implements the following changes for United Kingdom holidays:

• Implements Whit Monday holidays from 1871 onwards until its eventual replacement by Spring bank holidays in 1971 for England (ENG), Wales (WLS), and North Ireland (NIR).
• Add a cut-off date; thus, bank holidays are no longer available before the implementation of the Bank Holidays Act 1871.
• In-Lieu observance only took effect from 1875 onwards as decreed in the Bank Holidays Extension Act 1875.

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 7678311138

• 0 of 13 (100.0%) changed or added relevant lines in 2 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 7672154552:	0.0%
Covered Lines:	10440
Relevant Lines:	10440

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💛 - Coveralls

[KJhellico]

An excellent point about Whit Monday absence! 👍

[sonarqubecloud]

Quality Gate passed

The SonarCloud Quality Gate passed, but some issues were introduced.

1 New issue
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarCloud

[KJhellico]

LGTM!

[arkid15r]

🇬🇧 🇮🇲 LGTM