Allow user backends

[alex-oleshkevich]

This PR allows usage of custom user backends. Users can build their own backends and not be dependent on the shipped backends.

backend = MemoryBackend("")
async with Broadcast(backend=backend) as broadcast:
    async with broadcast.subscribe("chatroom") as subscriber:
        await broadcast.publish("chatroom", "hello")
        event = await subscriber.get()
        assert event.channel == "chatroom"
        assert event.message == "hello"

Also closes #90

[alex-oleshkevich]

@encode/maintainers anyone?

[tomchristie]

🐼 Can we test against public API?

[alex-oleshkevich]

We don't have public API for backends. If we reexport them in __init__.py then they will raise import errors because of missing dependencies. This is quite a big, unrelated change.
Or, alternatively, we can rename _backends -> backends making this public. What do you think?

[tomchristie]

Ah understood.

Given the intent of this PR perhaps the test could be against broadcaster.BroadcastBackend?

[alex-oleshkevich]

broadcaster.BroadcastBackend is an abstract class. Do you mean extending it for this particular case?

[tomchristie]

Yep.
For httpx we have a policy of only testing against the public API, which seems a good pattern.
We could be following this here also, right?

[tomchristie]

Looking good, thanks.
Couple of suggestions.

[tomchristie]

👍🏼