Fix possible error iterating on connection channels during concurrent…

… update

Fix for the crash reported in mosquito#589 unfortunately not tested to verify if
the fix is complete.

aio_pika/robust_connection.py

@@ -95,7 +95,9 @@ async def _on_connected(self) -> None:
             raise RuntimeError("No active transport for connection %r", self)
 
         try:
-            for channel in self.__channels:
+            # Make a copy of the channels to iterate on to guard from
+            # concurrent updates to the set.
+            for channel in list(self.__channels):
                 try:
                     await channel.restore()
                 except Exception: