Backporting fixes:
- Update workflow actions to latest versions #2447 by @jaheba
- Simplify setup.py #2525 by @jaheba
- Fix dataclass eventual handling. #2530 by @jaheba
- Improve README example #2538 by @lostella
- Change SeasonalNaive fallback predictor to nanmean #2549 by @abdulfatir
- Compact PandasDataset string representation #2558 by @lostella
- Update MANIFEST.in. #2566 by @jaheba
- Add NaN validation to Evaluator #2568 by @abdulfatir