Copy constructors

[Neil Schemenauer]

Guido van Rossum wrote:
> All I want is to disable changes to *existing* classes by default.
> You can write your own metaclass that changes the default, or you can
> inherit from a dynamic base class -- thus, with very little effort,
> you can make all your classes dynamic, if you want to.

This sounds reasonable to me.  In my experience, while assigning to
__class__ is cool, there is usually a good alternative that achieves the
same effect.

  Neil