Immutable types with mutable bases

The C API currently allows creating an immutable type with a mutable base: the type’s attributes are immutable, but inherited ones can be changed through the base.
The implications of this on cache invalidation are twisting my brain, and I doubt it’s useful for anyone, so I plan to deprecate it (and disable it in 3.14): gh-95388

  • Do you see a need for this? (In that case it’ll need proper tests.)
  • Do you know if it actually breaks something? If it never worked, it should probably be a disabled in 3.12.