Replace deprecated sklearn requirement

[itrharrison]

When running the tests for SOLikeT, a (intermittent!) failure happens because of the inherited dependency on sklearn with the following message:

      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

      More information is available at
      https://github.com/scikit-learn/sklearn-pypi-package

      If the previous advice does not cover your use case, feel free to report it at
      https://github.com/scikit-learn/sklearn-pypi-package/issues/new
      [end of output]

Here is me doing the third bullet point ;-)

[alessiospuriomancini]

thanks @itrharrison !

[shubhagrawal30]

This currently breaks installing through pip or using the tutorials. Marking a new release on PyPI and/or adding a git clone section to the readme and tutorials would be helpful.

[dpiras]

I think @alessiospuriomancini fixed this, could you please try to pip install cosmopower now @shubhagrawal30?