Fix deprecated decorator on classes that use super(...).__init__

In short, `__init__` here needs the same treatment as `__getstate__` and `__setstate__`.

Actually, *any* method on the class that uses `super()` internally is going to be affected by this issue, so unfortunately the only real way forward with this kind of implementation (which subclass's the original class, but with the same name) is going to suffer from this issue.

I can think of some possible workarounds but they could take some work to get just right, so I don't think it's important for now.  But use of `super()` in `__init__` is certainly common enough that it should be fixed for now.