Fix code to clear references to terminated threads

Author: gvanrossum

Lib/asyncio/unix_events.py

@@ -1381,8 +1381,8 @@ def _join_threads(self, timeout=None):
             thread.join(timeout)
 
         # Clear references to terminated threads
-        self._threads[:] = [thread for thread in list(self._threads.values())
-                            if thread.is_alive() and not thread.daemon]
+        self._threads = {key: thread for key, thread in self._threads.items()
+                         if thread.daemon or thread.is_alive()}
 
     def __enter__(self):
         return self