runtornado: Switch to asyncio event loop.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

zerver/lib/async_utils.py

@@ -0,0 +1,16 @@
+import asyncio
+
+
+class NoAutoCreateEventLoopPolicy(asyncio.DefaultEventLoopPolicy):  # type: ignore[misc,valid-type]  # https://github.com/python/typeshed/issues/7452
+    """
+    By default asyncio.get_event_loop() automatically creates an event
+    loop for the main thread if one isn't currently installed.  Since
+    Django intentionally uninstalls the event loop within
+    sync_to_async, that autocreation proliferates confusing extra
+    event loops that will never be run.  It is also deprecated in
+    Python 3.10.  This policy disables it so we don't rely on it by
+    accident.
+    """
+
+    def get_event_loop(self) -> asyncio.AbstractEventLoop:  # nocoverage
+        return asyncio.get_running_loop()