Update Slovenia holidays: add 2023 Solidarity Day

[kanobi]

Proposed change

This PR adds the "Solidarity Day" public holiday for Slovenia for August 14th 2023.
https://en.wikipedia.org/wiki/Public_holidays_in_Slovenia

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
• This PR is filed against beta branch of the repository
• This PR doesn't contain any merge conflicts and has clean commit history
• The code style looks good: make pre-commit
• All tests pass locally: make test, make tox (we strongly encourage adding tests to your code)
• The related documentation has been added/updated (check off the box for free if no updates is required)

[PPsyrius]

Some suggestions here :)

[coveralls]

Pull Request Test Coverage Report for Build 5854580446

• 2 of 2 (100.0%) changed or added relevant lines in 1 file 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 5788118002:	0.0%
Covered Lines:	9111
Relevant Lines:	9111

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💛 - Coveralls

[KJhellico]

LGTM.

[arkid15r]

Thanks for the contribution and prompt comments addressing @kanobi!

I've just merged it into beta (don't worry about the failed tests, it's unrelated to your PR)