Change sklearn to scikit-learn

[astro-nova]

Installing with pip throws an error:

"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: (...)

• replace 'sklearn' by 'scikit-learn' in your pip requirements files (requirements.txt, setup.py, setup.cfg, Pipfile, etc ...) (...) More information is available at https://github.com/scikit-learn/sklearn-pypi-package"

As far as I can see, this is the only place that uses sklearn instead of scikit-learn; after this change, the package installs fine. Feel free to disregard and sorry if this is wrong!

[robelgeda]

Thank you! I will take a look at why the CI is failing and merge this.