Suggestion: Implement MyPy type hints in the code

[YPCrumble]

Thanks for maintaining PyCronofy!

It would be awesome if this repo implemented MyPy type hints - currently I'm working on the generate_smart_invite code and it would be nice to have type hinting on the input parameters and also nice to understand the shape of the return value via the type hint.

If not possible, then it would be almost as helpful to include the return value in the docstring.

One more helpful thing would be to include a Python version of how to send the base64-encoded attachment in the smart invite email docs.