Rename __getinitargs__

• __getinitargs__ was used for pickling of old-style classes
• Keeping that name alive for hashing and pickling is dumb

The main thing here is that this needs to be done in a way that's backward compatible with Expression subclasses that only overload __getinitargs__.

cc @haogao2

That code should also complain if a class only overrides __getinitargs__.

mentioned in issue sumpy#33 (closed)