Make compatible with Tornado 5

Check whether or not the result of `WebSocketHandler.get` is awaitable or not before awaiting.

Author: rmorshea

jupyter_server_proxy/websocket.py

@@ -85,7 +85,10 @@ def undisallow(*args2, **kwargs2):
     async def get(self, *args, **kwargs):
         if self.request.headers.get("Upgrade", "").lower() != 'websocket':
             return await self.http_get(*args, **kwargs)
-        await super().get(*args, **kwargs)
+        else:
+            result = super().get(*args, **kwargs)
+            if inspect.isawaitable(result):
+                await result