Finding a class' superclasses

[Hallvard B Furuseth]

How do I find a class' immediate superclasses?
That is, given
  class A:       pass
  class B(A):    pass
  class C(A):    pass
  class D(B, C): pass
I want a function foo where foo(D) or foo(D()) returns (B, C).
(Well, the order of the classes is not important.)

-- 
Hallvard