Two nitpicky comments.

> Even if, right now, we were to change it to _note2, the maintainer of A could, in any future release of A, add a new _note2 variable which conflicts with something B is using.

This is why Python offers actual namespacing of properties and methods in the form of a double underscore (__). Double underscore prepends an underscore and the class name to the private property name which prevents these sorts of collisions.

  class Bprime(object):
     def __init__(self, a):
         for var in dir(a):
            setattr(self, var, getattr(a, var))

> Uh oh. Looks like composition is worse than inheritance.

This is not composition.