validate that all async methods are actually async

[komuw]

if we have something like;

class Task:
    async def async_run(self, *args, **kwargs):
        raise NotImplementedError("run method must be implemented.")

and we expect users to implement the async def async_run method; we should - to the best extent that python affords us - validate that the users implementation is actually async.

[komuw]

superseded by: #4