keep send side open while receiving in tcp-p3 example

Netcat likes to close the connection if the receive half is closed by the remote
host, regardless of whether stdin has been closed, and there's no obvious way to
change that behavior.  So on the client side we hold both halves open until
we've received something, which matches the p2 example behavior.

Author: dicej

examples/tcp-p3/app.py

@@ -69,13 +69,12 @@ async def send_and_receive(address: IPAddress, port: int) -> None:
 
     send_tx, send_rx = wit_world.byte_stream()
     async def write() -> None:
-        with send_tx:
-            await send_tx.write_all(b"hello, world!")
-    await asyncio.gather(sock.send(send_rx), write())
+        await send_tx.write_all(b"hello, world!")
 
     recv_rx, recv_fut = sock.receive()
     async def read() -> None:
         with recv_rx:
             data = await recv_rx.read(1024)
             print(f"received: {str(data)}")
-    await asyncio.gather(recv_fut.read(), read())
+            send_tx.__exit__(None, None, None)
+    await asyncio.gather(recv_fut.read(), read(), sock.send(send_rx).read(), write())