A start on the data model of Jython 3.8

I put a PR on the official repo that reflects ideas I tested in the Very 
Slow Jython project and briefed at the language summit.

https://github.com/jython/jython/pull/107

It's been there a month without adverse comment, and gained a few emojis 
of approval, so I'll merge this coming week if there are no last minute 
objections. (And there's always a way back.)

I'll put up another tranche of work as a PR after that.

Jeff

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Jeff Allen