Fixed #30498 -- Fixed proxy class caching in lazy().

lazy() should prepare the proxy class only once (the first time it's
used) not on every call.

Regression in b4e76f30d1.

django/utils/functional.py

@@ -79,7 +79,7 @@ def lazy(func, *resultclasses):
             self.__kw = kw
             if not self.__prepared:
                 self.__prepare_class__()
-            self.__prepared = True
+            self.__class__.__prepared = True
 
         def __reduce__(self):
             return (