Fix incorrect return type annotations in darts.utils.statistics

[AlejandroCoronadoN]

Three functions in darts/utils/statistics.py have return type annotations that do not match what they actually return:

• stationarity_test_kpss is annotated -> set but returns a tuple (it wraps statsmodels.tsa.stattools.kpss).
• stationarity_test_adf is annotated -> set but returns a tuple (it wraps adfuller).
• granger_causality_tests is annotated -> None but returns a dict (it wraps grangercausalitytests); its own docstring already says it returns a Dict.

The docstrings and the existing tests already treat these as tuples/dict (for example stationarity_test_kpss(series)[1]), so the annotations are just wrong and mislead type checkers and IDEs. This fixes the annotations to match the real return values and adds a small regression test.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.06%. Comparing base (4ce4d4b) to head (77e065f).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3185   +/-   ##
=======================================
  Coverage   97.06%   97.06%           
=======================================
  Files         169      169           
  Lines       18898    18898           
=======================================
  Hits        18343    18343           
  Misses        555      555           

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[dennisbader]

Thanks a lot for the PR and fix @AlejandroCoronadoN 🚀

I removed the tests (not really necessary to check the return type) and slightly adapted the changelog entry.

I'll merge once all tests have passed.