List optional cytoolz dependency

[jakirkham]

Would be nice if somehow cytoolz was noted as an optional dependency in setup.py and/or requirements.txt. Not totally sure the right place to add it though.

[mrocklin]

I would trust your judgement on how best to handle optional dependencies.

[rbubley]

@jakirkham – one way would be:
https://setuptools.readthedocs.io/en/latest/setuptools.html#declaring-extras-optional-features-with-their-own-dependencies

[GenevieveBuckley]

The page Russ linked to appears to have moved. I think this might be where it is now? https://setuptools.pypa.io/en/latest/userguide/dependency_management.html?highlight=optional%20features%20with%20their%20own%20dependencies#optional-dependencies