Tornado WebSocketHandler fixup (#1083)

* Tornado WebSocketHandler fixup

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

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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

jupyter_server/base/zmqhandlers.py

@@ -2,6 +2,8 @@
 # Raise a warning that this module is deprecated.
 import warnings
 
+from tornado.websocket import WebSocketHandler
+
 from jupyter_server.base.websocket import WebSocketMixin
 from jupyter_server.services.kernels.connection.base import (
     deserialize_binary_message,

jupyter_server/services/kernels/connection/channels.py

@@ -281,7 +281,7 @@ async def _register_session(self):
         stale_handler = self._open_sessions.get(self.session_key)
         if stale_handler:
             self.log.warning("Replacing stale connection: %s", self.session_key)
-            await stale_handler.close()
+            stale_handler.close()
         if (
             self.kernel_id in self.multi_kernel_manager
         ):  # only update open sessions if kernel is actively managed