nicely_exit doesn't exit nicely with python >=3.7

From Python 3.7, asyncio.Task.all_tasks() has been moved to asyncio.all_tasks() and asyncio.Task.current_task() has moved to asyncio.current_task()

Therefore, nicely_exit of autobahn/asyncio/component.py doesn't exit so nicely anymore

    @asyncio.coroutine
    def nicely_exit(signal):
        log.info("Shutting down due to {signal}", signal=signal)

        tasks = asyncio.Task.all_tasks()
        for task in tasks:
            # Do not cancel the current task.
            if task is not asyncio.Task.current_task():
                task.cancel()

and throws the following exception

AttributeError: type object '_asyncio.Task' has no attribute 'all_tasks'

Python >= 3.7
Autobahn 20.7.1

Start in autobahn/asyncio/component.py at the nicely_exit function referenced in the issue. Check the asyncio task APIs used there against Python 3.7 and verify that shutdown completes without the reported AttributeError. Done means nicely_exit handles Python 3.7 and later without raising this exception.