Type annotations

[dsynkd]

Python has added support for type hints since PEP 483, and I want to implement this into the source code. Type systems are of course a highly debated topic among programmers, I believe type annotations would help with readability and type checking would help with finding errors. Let me know if this is something that the community would be receptive towards.

[djmattyg007]

We're already using mypy in the main Mopidy codebase:

https://github.com/mopidy/mopidy/blob/develop/tox.ini#L31

I think rolling this out to Mopidy extensions (especially popular ones such as this) would be greatly appreciated 👍