Always close iopub socket

We sometime get

```
 FAILED tests/test_connect.py::test_port_bind_failure_gives_up_retries - pytest.PytestUnraisableExceptionWarning: Exception ignored in: <function Socket.__del__ at 0x7efe22e4d760>

  Traceback (most recent call last):
    File "/opt/hostedtoolcache/Python/3.12.9/x64/lib/python3.12/site-packages/zmq/sugar/socket.py", line 185, in __del__
      warn(
  ResourceWarning: Unclosed socket <zmq.asyncio.Socket(zmq.PUB) at 0x7efe1eb32820><F48>
```

The only zmq.PUB I can find is iopub, and we don't seem to be closing it

In addition : should app be a contextmanager ?
And should the atexit close registring be replaced with a weakref
finalizer ?
t

Author: Carreau

ipykernel/kernelapp.py

@@ -427,7 +427,7 @@ def close(self):
         if self.debug_shell_socket and not self.debug_shell_socket.closed:
             self.debug_shell_socket.close()
 
-        for channel in ("shell", "control", "stdin"):
+        for channel in ("shell", "control", "stdin", "iopub"):
             self.log.debug("Closing %s channel", channel)
             socket = getattr(self, channel + "_socket", None)
             if socket and not socket.closed: