how to tell if it's safe to go from new-style to classic class?

[Skip Montanaro]

If I have a new-style class what are the restrictions that keep me from
going back to a classic class?  I know use of __slots__, __new__ or the
property() builtin make new-style a requirement.  What other constructs
might the code be using that would prevent me from falling back to use of
classic classes?

Thx,

Skip