Type hint using stub file instead of inline comments

[GhostofGoes]

Move to using a *.pyi stub file for type hints instead of the current inline comments. There are a number of reasons I'd like to do this:

• Ability to use complex types without adding more imports, as well as recent language features like forward references and TypedDict on a codebase that presently still supports 2.7
• Readability: type comments don't look great and clutter the already ugly (in my opinion) code base
• Performance: avoid attempting to import the typing module