Replace HolidayBase::country/market with HolidayBase::code

[arkid15r]

Proposed change

Another step in out v1 refactoring journey. Here I replaced HolidayBase::country and HolidayBase::market attributes with unified HolidayBase::code. I also added HolidayBase::scope to be able to identify entity's type easily.

In order to satisfy tests there is an unrelated change of HolidayBase::__repr__ that requires more opinions (keeping it that way for now).

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)
• [x] Breaking change (a code change causing existing functionality to break)
• [ ] New feature (new python-holidays functionality in general)

Checklist

• [x] I've followed the contributing guidelines
• [x] I've run make pre-commit, it didn't generate any changes
• [x] I've run make test, all tests passed locally

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (79af62f) to head (7f9fe3d). Report is 8 commits behind head on v1.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## v1 #1865 +/- ## ========================================== Coverage 100.00% 100.00% ========================================== Files 182 184 +2 Lines 10473 10609 +136 Branches 1726 1719 -7 ========================================== + Hits 10473 10609 +136 ```

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