Backport PR #669: Keep preferring SelectorEventLoop on Windows

Author: blink1073

ipykernel/kernelapp.py

@@ -516,14 +516,21 @@ def _init_asyncio_patch(self):
         Pick the older SelectorEventLoopPolicy on Windows
         if the known-incompatible default policy is in use.
 
+        Support for Proactor via a background thread is available in tornado 6.1,
+        but it is still preferable to run the Selector in the main thread
+        instead of the background.
+
         do this as early as possible to make it a low priority and overrideable
 
         ref: https://github.com/tornadoweb/tornado/issues/2608
 
-        FIXME: if/when tornado supports the defaults in asyncio,
-               remove and bump tornado requirement for py38
+        FIXME: if/when tornado supports the defaults in asyncio without threads,
+               remove and bump tornado requirement for py38.
+               Most likely, this will mean a new Python version
+               where asyncio.ProactorEventLoop supports add_reader and friends.
+
         """
-        if sys.platform.startswith("win") and sys.version_info >= (3, 8) and tornado.version_info < (6, 1):
+        if sys.platform.startswith("win") and sys.version_info >= (3, 8):
             import asyncio
             try:
                 from asyncio import (