Context manager API for EndpointAPI.subscribe

[pipermerriam]

What was wrong?

The EndpointAPI.subscribe API returns a handle on the subscription that the user can call to remove this subscription. This is easy to forget or in some contexts, not always easy have a clear place where this would be called.

How can it be fixed?

We can fix this by making lahja.common.Subscription a context manager and having it automatically call Subscription.unsubscribe in the __exit__. This allows for the subscription API to be used like this:

with EndpointAPI.subscribe(MyEvent, handler_fn):
    # subscription active here:
# automatically unsubscribed here