Adapt to Pandas 3.0.0+

[popescu-v]

Pandas 3.0.0 has been released on January 21st, 2026. Hence, all environments that use Python >= 3.11 pull Pandas 3.0.0 as a dependency of khiops-python.
However, Pandas 3.0.0 contains breaking changes with respect to previous versions, including Pandas 2.3.3. Most notably, the StringDType is used by default, rather than object. As the StringDType is a Pandas data type, as opposed to object which is a NumPy data type, all code assuming a by-default Pandas / NumPy compatibility regarding the Pandas column types breaks now. For example, code like np.issubdtype(my_pandas_dtype, numpy_dtype) breaks if my_pandas_dtype is a StringDType, with TypeError: Cannot interpret '<StringDtype(storage='python', na_value=nan)>' as a data type.