The problem comes when PyErr_SetObject triggers garbage collection which
runs python code (finalizers...).

I could reproduce the crash several times, and each time garbage
collection was triggered by the normalization of the exception (the call
to PyEval_CallObject(exception, args)).

An obvious fix is to save exc_value near the "/* Implicit exception
chaining */" comment.

However, I fear that there are other places where exc_value is reset,
and "exception chaining" may break in subtle ways.