Prevent RuntimeError while reinitializing clusters

Author: agronholm

redis/anyio/cluster.py

@@ -1290,7 +1290,7 @@ async def initialize(self) -> None:
         startup_nodes_reachable = False
         fully_covered = False
         exception = None
-        for startup_node in self.startup_nodes.values():
+        for startup_node in tuple(self.startup_nodes.values()):
             try:
                 # Make sure cluster mode is enabled on this node
                 try: