Regression in test_shutdown on macOS

There appears to be a regression that happened between 5.1.2 and 5.1.3. The `test_shutdown` had become flaky. Bisecting this behaviour seems to point to d4d870c3229ef29116f7b37985de9229be86767f as the first bad commit. For some reason the problem is only seen on python 3.7 and not python 3.8.
```
________________________________ test_shutdown _________________________________

def test_shutdown():
"""Kernel exits after polite shutdown_request"""
with new_kernel() as kc:
km = kc.parent
> execute(u'a = 1', kc=kc)

ipykernel/tests/test_kernel.py:320:
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
ipykernel/tests/utils.py:68: in execute
busy = kc.get_iopub_msg(timeout=TIMEOUT)
/nix/store/a19wr1bwvf12x34lgm0izcvd5hrpcsl4-python3.7-jupyter_client-5.3.4/lib/python3.7/site-packages/jupyter_client/client.py:81: in get_iopub_msg
return self.iopub_channel.get_msg(*args, **kwargs)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

self =
block = True, timeout = 15000

def get_msg(self, block=True, timeout=None):
""" Gets a message if there is one that is ready. """
if block:
if timeout is not None:
timeout *= 1000 # seconds to ms
ready = self.socket.poll(timeout)
else:
ready = self.socket.poll(timeout=0)

if ready:
return self._recv()
else:
> raise Empty
E _queue.Empty

/nix/store/a19wr1bwvf12x34lgm0izcvd5hrpcsl4-python3.7-jupyter_client-5.3.4/lib/python3.7/site-packages/jupyter_client/blocking/channels.py:57: Empty
```