[pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

Author: pre-commit-ci[bot]

ipykernel/kernelapp.py

@@ -738,7 +738,7 @@ def start(self) -> None:
             self.poller.start()
         backend = "trio" if self.trio_loop else "asyncio"
         run(self.main, backend=backend)
-        return
+        return None
 
     async def _wait_to_enter_eventloop(self):
         await self.kernel._eventloop_set.wait()