Add async for support for consuming messages

[wallyqs]

Implement asynchronous iterator for the client: https://www.python.org/dev/peps/pep-0492/#asynchronous-iterators-and-async-for

sub = await sc.subscribe("foo")
async for msg in sub.messages:
    print("Received a message (seq={}): {}".format(msg.seq, msg.data))

[1755]

Is anyone doing the issue? I can try to open PR to solve this

[wallyqs]

not that I'm aware of, @1755 feel to send PR and will take a look 👍

[wallyqs]

This is included in v1-dev branch already