fix: sklearn -> scikit-learn

[zzc0430]

The 'sklearn' PyPI package is deprecated, use 'scikit-learn' rather than 'sklearn' for pip commands.

Here is how to fix this error in the main use cases:

• use 'pip install scikit-learn' rather than 'pip install sklearn'
• replace 'sklearn' by 'scikit-learn' in your pip requirements files (requirements.txt, setup.py, setup.cfg, Pipfile, etc ...)
• if the 'sklearn' package is used by one of your dependencies, it would be great if you take some time to track which package uses 'sklearn' instead of 'scikit-learn' and report it to their issue tracker
• as a last resort, set the environment variable SKLEARN_ALLOW_DEPRECATED_SKLEARN_PACKAGE_INSTALL=True to avoid this error