Asynchronous task iterator as argument to Executor.submit()

[azawlocki]

Current status: seems to work, needs polishing, more testing, a convincing example and documentation.

Update: CI fails due to

• For Python 3.6: no asyncio.run() function present in asyncio (can be fixed easily by replacing it with asyncio.get_event_loop().run_until_complete()
• Wrong license for the aioconsole package this branch depends on

Update: Ready for review.

Added an integration test. An excerpt:

        # We use an async task generator that yields tasks removed from
        # an async queue. Each computed task will potentially spawn
        # new tasks -- this is made possible thanks to using async task
        # generator as an input to `executor.submit()`.

        task_queue = asyncio.Queue()

        # Seed the queue with the first task:
        await task_queue.put(Task(data=3))

        async def input_generator() -> AsyncGenerator[Task]:
            """Task generator yields tasks removed from `queue`."""
            while True:
                task = await task_queue.get()
                if task.data == 0:
                    break
                yield task

        async for task in executor.submit(worker, input_generator()):
            print("task result:", task.result, file=sys.stderr)
            for n in range(task.result):
                await task.queue.put(Task(data=task.result - 1))

        print("all done!", file=sys.stderr)

[shadeofblue]

looks good as far as I could tell