Remove constraint on `scikit-learn <1.6.0` due to `category-encoders` incompatibility with Python 3.9

[guillaume-vignal]

I would like to request the removal of the constraint on scikit-learn <1.6.0 in the project. Currently, this constraint exists due to the incompatibility between category-encoders and scikit-learn >=1.6.0 when using Python 3.9. However, the maintainers of category-encoders have stated that they do not plan to release a version compatible with Python 3.9 and scikit-learn >=1.6.0.

Given this situation, we are effectively blocked from upgrading scikit-learn while still supporting Python 3.9.

When the support of python 3.9 will be over, we'll be able to remove the constraint on scikit-learn <1.6.0.