I think the comment described "Why Py_ALLOW_RECURSION is required", not "t may be NULL even if s is in the intern dict".

> If PyDict_GetItem fails due to stack overflow, perhaps the Python process is doomed to fail soon anyway.

When I changed the code to use PyDict_SetDefault(), I found "USE_STACKCHECK" macro.
See https://docs.python.org/3/c-api/sys.html#c.PyOS_CheckStack

But I don't know this can be really happend. Interned dict only contains exact unicode objects. There is no chance for
calling user defined __hash__() or __eq__().