Fix utils.set_index_dtypes()

[hagenw]

Ensures audformat.utils.set_index_dtypes() is able to change a string dtype from str (pd.StringDtype(na_value=np.nan)) to string (pd.StringDtype(na_value=pd.NA)) as well.

As set_index_dtypes() works again, we can switch back to the original code of segmented_index(). The new code shows identical behavior, but it is nice to limit the changes in dev to the once needed to support pandas 3.0.0.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (6e48937) to head (cd8e320).
⚠️ Report is 1 commits behind head on dev.

Additional details and impacted files

Files with missing lines	Coverage Δ
audformat/core/index.py	100.0% <100.0%> (ø)
audformat/core/utils.py	100.0% <100.0%> (ø)

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