bpo-28866: No type cache for types with specialized mro, invalidation is hard. (#13157)

* No type cache for types with specialized mro, invalidation is hard.

* FIX: Don't disable method cache custom types that do not implement mro().

* fixing implem.

* Avoid storing error flags, also decref.

* news entry

* Clear as soon as we're getting an error.

* FIX: Reference leak.