Support pickling

[anler]

Hello,

I've seen you added support for pickling, but it does not take into account Mutable._parents which is WeakKeyDict that cannot be pickled, see Supporting Pickling in SQLAlchemy.

Your pickling tests pass because Mutable._parents is memoized property so it won't create the dict until you access it. See these changes I've made and that cause the test to fail: anler@a36b612

Thanks!