Fix sklearn version requirement to avoid import error.

[anntis]

Description

DML imports methods from scikit-learn, notably, root_mean_squared_error, which is implemented starting from scikit-learn version 1.4.
File requirements.txt for DML requires scikit-learn version < 1.6.0 which in some environments installs scikit-learn earlier than 1.4 causing an Import Error.
This PR refines the requirement so that scikit-learn version must be between 1.4.0 (including) and 1.6.0 (excluding).

Reference to Issues or PRs

#279

PR Checklist

• The title of the pull request summarizes the changes made.
• The PR contains a detailed description of all changes and additions.
• References to related issues or PRs are added.
• The code passes all (unit) tests.
• Enhancements or new feature are equipped with unit tests.
• The changes adhere to the PEP8 standards.