[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

@@ -734,12 +734,12 @@ def start(self) -> None:
         """Start the application."""
         if self.subapp is not None:
             self.subapp.start()
-            return None
+            return
         if self.poller is not None:
             self.poller.start()
         backend = "trio" if self.trio_loop else "asyncio"
         run(self.main, backend=backend)
-        return None
+        return
 
     async def _wait_to_enter_eventloop(self):
         await self.kernel._eventloop_set.wait()