Support Pandas 2.1

[celestinoxp]

WARNING:C:\Users\username\anaconda3\lib\site-packages\category_encoders\utils.py:50: FutureWarning: is_categorical_dtype is deprecated and will be removed in a future version. Use isinstance(dtype, CategoricalDtype) instead
return pd.api.types.is_categorical_dtype(dtype)

Windows x64
Python 3.10
scikit-learn 1.3
pandas 2.1

[PaulWestenthanner]

fixed by #419