Add type information (PEP 561)

[SimonDR-Boltzmann]

It would be nice if static type checkers like mypy could pick up the types of this package (cfr. https://mypy.readthedocs.io/en/stable/installed_packages.html#creating-pep-561-compatible-packages)

[briancurtin]

I don't want to add the old comment style way since this still supports 2.x, so the stub method would be what we'd do to solve this. I've only ever done in-line annotations so I don't have any direct experience with stubs, so PRs welcome if someone wants to work on it before I get to it.

[9y2070m]

I would need the type hints too. Maybe I can provide you with a stubs file.