gh-105699: Fix an Interned Strings Crasher (gh-106930)

gh-105699: Fix an Interned Strings Crasher (gh-106930)

A static (process-global) str object must only have its "interned" state cleared when no longer interned in any interpreters.  They are the only ones that can be shared by interpreters so we don't have to worry about any other str objects.

We trigger clearing the state with the main interpreter, since no other interpreters may exist at that point and _PyUnicode_ClearInterned() is only called during interpreter finalization.

We do not address here the fact that a string will only be interned in the first interpreter that interns it.  In any subsequent interpreters str.state.interned is already set so _PyUnicode_InternInPlace() will skip it.  That needs to be addressed separately from fixing the crasher.