Drop PyPy support

[arkid15r]

Proposed change

Originally the PR was Update test matrix: remove pypy. After a talk w/ @KJhellico we agreed to remove it from the package metadata too. Removing this code simplifies maintenance process a lot.

All pypy related issues (not sure if we've ever seen one) will still be handled as any other bug fix requests -- on individual basis.

Type of change

• [ ] New country/market holidays support (thank you!)
• [ ] Supported country/market holidays update (calendar discrepancy fix, localization)
• [x] 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

• [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

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov[bot]]

Codecov Report

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

Project coverage is 100.00%. Comparing base (8d821de) to head (a77ef37).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## dev #1878 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 184 184 Lines 11194 11194 Branches 1765 1765 ========================================= Hits 11194 11194 ```

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