Cannot save pydef-defined class

```julia
@pydef type Store
__init__(self, a) = (self[:a] = a)
end
save("dummy.jld", "x", Store(10))
> PyError (ccall(@pysym(:PyObject_Call), PyPtr, (PyPtr, PyPtr, PyPtr), o, arg, C_NULL))
> PicklingError("Can't pickle : attribute lookup __main__.Store failed",)
```

I don't think anything can be done about it, though.

Start by reproducing the failure from the issue's @pydef Store example and the save("dummy.jld", "x", Store(10)) call. Investigate how PyCallJLD handles pydef-defined classes and cPickle serialization; done would require either successful saving and loading of the object or a clearly documented supported limitation.

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