Make datasets PEP-561 compliant
This is super cool, I love that ❤️ 
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
