Asyncio watcher 'MultiLoopChildWatcher' currently doesn't work well
on Solaris and can freeze the event loop indefinitely.

This was reported upstream:
https://bugs.python.org/issue37573

--- Python-3.9.0/Lib/asyncio/unix_events.py
+++ Python-3.9.0/Lib/asyncio/unix_events.py
@@ -30,7 +30,7 @@ __all__ = (
     'SelectorEventLoop',
     'AbstractChildWatcher', 'SafeChildWatcher',
     'FastChildWatcher', 'PidfdChildWatcher',
-    'MultiLoopChildWatcher', 'ThreadedChildWatcher',
+    'ThreadedChildWatcher',
     'DefaultEventLoopPolicy',
 )