Subject should implement AsyncContextManager for auto-unsubscribe

[skewty]

Proposed change

class Subject:
    async def __aenter__(self):
        return self

    async def __aexit__(self, *args, **kwargs):
        await self.unsubscribe()

Use case

async with AsyncExitStack() as stack:
    nc = await nats.connect()
    await stack.enter_async_context(nc)
    await stack.enter_async_context(await nc.subscribe(...))
    await stack.enter_async_context(await nc.subscribe(...))

Contribution

No response

[caspervonb]

On Subscription? but yes, pull request very welcome 👍