sum() requires number, not simply __add__

[Buck Golemon]

I feel like the design of sum() is inconsistent with other language
features of python. Often python doesn't require a specific type, only
that the type implement certain methods.

Given a class that implements __add__ why should sum() not be able to
operate on that class?

We can fix this in a backward-compatible way, I believe.

Demonstration:
    I'd expect these two error messages to be identical, but they are
not.

     >>> class C(object): pass
     >>> c = C()
     >>> sum((c,c))
    TypeError: unsupported operand type(s) for +: 'int' and 'C'
    >>> c + c
    TypeError: unsupported operand type(s) for +: 'C' and 'C'