bpo-43498: Fix dictionary iteration error in _ExecutorManagerThread (GH-24868)

(cherry picked from commit 7431448b817d3bf87f71661cf8f3d537807ab2e2)

Co-authored-by: Jakub KulĂ­k <Kulikjak@gmail.com>
diff --git a/Lib/concurrent/futures/process.py b/Lib/concurrent/futures/process.py
index 7647198..6ee2ce6 100644
--- a/Lib/concurrent/futures/process.py
+++ b/Lib/concurrent/futures/process.py
@@ -373,7 +373,7 @@ def wait_result_broken_or_wakeup(self):
         assert not self.thread_wakeup._closed
         wakeup_reader = self.thread_wakeup._reader
         readers = [result_reader, wakeup_reader]
-        worker_sentinels = [p.sentinel for p in self.processes.values()]
+        worker_sentinels = [p.sentinel for p in list(self.processes.values())]
         ready = mp.connection.wait(readers + worker_sentinels)
 
         cause = None