ReferenceError is now built-in, so pick it up from the right place.

It still needs to be here to preserve the API.

Lib/weakref.py

@@ -16,11 +16,13 @@ from _weakref import \
      getweakrefs, \
      ref, \
      proxy, \
-     ReferenceError, \
      CallableProxyType, \
      ProxyType, \
      ReferenceType
 
+from exceptions import ReferenceError
+
+
 ProxyTypes = (ProxyType, CallableProxyType)
 
 __all__ = ["ref", "proxy", "getweakrefcount", "getweakrefs",