Add SubscriptionAPI

And `TrioSubscriptionAPI`, to make subscription io-agnostic.

tests/pubsub/test_subscription.py

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+import math
+
+import pytest
+import trio
+
+from libp2p.pubsub.pb import rpc_pb2
+from libp2p.pubsub.subscription import TrioSubscriptionAPI
+
+GET_TIMEOUT = 0.001
+
+
+def make_trio_subscription():
+    send_channel, receive_channel = trio.open_memory_channel(math.inf)
+    return send_channel, TrioSubscriptionAPI(receive_channel)
+
+
+def make_pubsub_msg():
+    return rpc_pb2.Message()
+
+
+async def send_something(send_channel):
+    msg = make_pubsub_msg()
+    await send_channel.send(msg)
+    return msg
+
+
+@pytest.mark.trio
+async def test_trio_subscription_get():
+    send_channel, sub = make_trio_subscription()
+    data_0 = await send_something(send_channel)
+    data_1 = await send_something(send_channel)
+    assert data_0 == await sub.get()
+    assert data_1 == await sub.get()
+    # No more message
+    with pytest.raises(trio.TooSlowError):
+        with trio.fail_after(GET_TIMEOUT):
+            await sub.get()
+
+
+@pytest.mark.trio
+async def test_trio_subscription_iter():
+    send_channel, sub = make_trio_subscription()
+    received_data = []
+
+    async def iter_subscriptions(subscription):
+        async for data in sub:
+            received_data.append(data)
+
+    async with trio.open_nursery() as nursery:
+        nursery.start_soon(iter_subscriptions, sub)
+        await send_something(send_channel)
+        await send_something(send_channel)
+        await send_channel.aclose()
+
+    assert len(received_data) == 2
+
+
+@pytest.mark.trio
+async def test_trio_subscription_cancel():
+    send_channel, sub = make_trio_subscription()
+    await sub.cancel()
+    # Test: If the subscription is cancelled, `send_channel` should be broken.
+    with pytest.raises(trio.BrokenResourceError):
+        await send_something(send_channel)
+    # Test: No side effect when cancelled twice.
+    await sub.cancel()
+
+
+@pytest.mark.trio
+async def test_trio_subscription_async_context_manager():
+    send_channel, sub = make_trio_subscription()
+    async with sub:
+        # Test: `sub` is not cancelled yet, so `send_something` works fine.
+        await send_something(send_channel)
+    # Test: `sub` is cancelled, `send_something` fails
+    with pytest.raises(trio.BrokenResourceError):
+        await send_something(send_channel)