Lars, I think that your question has a simple explanation, and is not
related to the original issue:
- On 4th line, the variable named "A" is bound to a new type object.
This type has no attribute by itself.
- The first dir(A) displays the attributes of the bases class, which are
the previous A and B: you see ['one', 'two']
- The second dir(A) displays the attributes of the bases class, which
are B and C: you see ['two', 'three']

The instruction "A = type(...)" causes A to refer to another object; it
does not change the object previously contained in A.