Don't use mutables in signatures

(cherry picked from commit a88706142f)

game/scripts/interface/book.rpy

@@ -1,12 +1,12 @@
 init python:
     class book_readable_class(object):
-        def __init__(self, title, contents=[], **kwargs):
+        def __init__(self, title, contents=(), **kwargs):
             self.title = title
             self.page = 0
             self.overflow=None
 
             self.title = title
-            self.contents = contents
+            self.contents = list(contents)
             self.__dict__.update(**kwargs)
 
             self.pages = len(self.contents)-1