Python Asynchronous I/O Support

[ChihweiLHBird]

It would be nice to support Python's asyncio for provider and application authors because it's faster than multithreading for I/O, e.g. API request made to the backend server of the feature flags.

[federicobond]

Sure! Speaking for myself as a maintainer I don't have much experience with async in Python.

Would you be willing to hash out the details and/or provide a proof of concept for how it would work? I can help with review.

[ChihweiLHBird]

@federicobond Thank you! I can give it a try at a later point.

[gruebel]

Implemented via #413