Make datasets PEP-561 compliant
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This is super cool, I love that ❤️ 
2021-05-28T13:10:10Z
https://github.com/huggingface/datasets/pull/2417#issuecomment-850407988
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Allows to type-check datasets with `mypy` when imported as a third-party library

PEP-561: https://www.python.org/dev/peps/pep-0561
MyPy doc on the subject: https://mypy.readthedocs.io/en/stable/installed_packages.html
