Make __class__ assignment possible, when the object structures are the

same.  I hope the test for structural equivalence is stringent enough.
It only allows the assignment if the old and new types:

- have the same basic size
- have the same item size
- have the same dict offset
- have the same weaklist offset
- have the same GC flag bit
- have a common base that is the same except for maybe the dict and
  weaklist (which may have been added separately at the same offsets
  in both types)