Pickling and unpickling inherited attributes

[Alex]

Thanks to both Alex Martelli and Steve Holden.. We need __slots__ for
other reasons, too long to explain, but basically to prevent assignment
of extra attributes.

I ended up changing child classes this way:

    def __getstate__(self):
        return(Parent.__getstate__(self), self.C)
    def __setstate__(self, data):
        Parent.__setstate__(self, data[0])
        self.C=data[1:]

This seems to work fine.