bpo-35491, multiprocessing: replace "RUN" with RUN (GH-11178)

Author: vstinner

Lib/multiprocessing/pool.py

@@ -486,7 +486,7 @@ def _handle_results(outqueue, get, cache):
                 util.debug('result handler got EOFError/OSError -- exiting')
                 return
 
-            if thread._state != "RUN":
+            if thread._state != RUN:
                 assert thread._state == TERMINATE, "Thread not in TERMINATE"
                 util.debug('result handler found thread._state=TERMINATE')
                 break