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It sounds like this bug is another case where we have made some objects
per-interpreter but others are still global and this is causing problems.
_PyUnicode_EqualToASCIIId() wouldn't have any problems if interpreters
weren't sharing any objects (or were only sharing immutable "immortal"
objects).
For now can we just move the relevant per-interpreter strings from
PyInterpreterState.unicode_state ("interned" and "ids") up to
_PyRuntimeState. They will then be global, which should restore 
the correct behavior.
Personally, I'd rather we not revert the original change. Moving the data
to _PyRuntimeState would save me some effort with related work I'm doing
right now.
Of course, the potential bug would still exist in _PyUnicode_EqualToASCIIId().
Could we add a test as part of this fix to verify the failure case described
here actually works?