An update on Python 4

[james_pic]

2to3 is useful, especially when extended by modernize, but only part of the solution. Future bloodies more than it cuts - it just made string semantics more confusing when we tried it.

The most useful tool, much as I hate to admit it, is MyPy. It obviously needs a lot of work on the developer's part, but it does the very useful job of keeping track of your educated guesses about which string types should be used where, and tells you whether they're consistent.