Avoid circular reference

(cherry picked from commit db30cd66d3)

game/scripts/interface/letter.rpy

@@ -180,7 +180,10 @@ init python:
             self.wait = wait
             self.label = label
             self.func = func
-            self.queue = mailbox.letters
+
+        @property
+        def queue(self):
+            return mailbox.letters
 
         def send(self):
             self.mailed = True