Aronnem cleanup

The important change here is probably the corrected explanation for `is not` (which is a different test than `!=`, they only give the same for small integers.). The other stuff is perhaps debatable. It might be difficult to try to correctly describe how `is` actually works to python beginners, so the extra stuff I added about `is` might not be helpful.

I didn't organize my changes here so there is probably too much stuff piled together - if there is some helpful way to split out the useful changes, let me know.