set_event_loop_policy for windows

# patch c.f. tornadoweb/tornado#2608 (comment)
asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())  # python-3.8.x

Author: juhuebner

nbclient/client.py

@@ -32,6 +32,10 @@
 from .output_widget import OutputWidget
 
 
+# patch c.f. https://github.com/tornadoweb/tornado/issues/2608#issuecomment-491489432
+asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())  # python-3.8.x
+
+
 def timestamp() -> str:
     return datetime.datetime.utcnow().isoformat() + 'Z'