Add type hinting to the code base and encourage new contributions to include them

[Jawfish]

CircuitPython and KMK's target Python version, 3.6, both support type hinting. I feel this would greatly improve the developer experience and make it easier for new contributors to dive in and familiarize themselves with the codebase.

Some benefits:

• Most modern code editors that support Python allow quick navigation to type definitions, so contributors can easily see exactly what any given object is and does
• Catch more errors in the IDE - especially beneficial given this is an embedded systems project
• IntelliSense and autocomplete for easier familiarization with the codebase and prevention of typos or accessing incorrect/non-existent properties

I volunteer to begin undertaking this if maintainers approve of it.

[Jawfish]

Nevermind, just saw #205