Avoid circular reference

it clogs pickling and garbage-collecting

(cherry picked from commit 42f690f966)

game/scripts/interface/parcel.rpy

@@ -1,4 +1,3 @@
-
 init python:
     class Parcel(object):
         """
@@ -18,7 +17,10 @@ init python:
             self.wait = wait
             self.label = label
             self.func = func
-            self.queue = mailbox.parcels
+
+        @property
+        def queue(self):
+            return mailbox.parcels
 
         def send(self):
             self.mailed = True