gh-128078: clear caller's StopAsyncIteration in _PyGen_SetStopIterationValue

[picnixz]

Some calls have a StopAsyncIteration exception set at this point but we should ignore them since this function is meant to create a StopIteration instead (or replace an existing StopAsyncIteration)

Alternative I have also envisaged:

• Unconditionally clear an optional caller's exception: this would be probably fine but this could hide bugs.
• Also clear StopIteration: strictly speaking I think we could also do it but as I'm not extremely familiar with the call graph, I'd prefer restricting it to StopAsyncIteration for now.

• Issue: Async generator/anext with default-tuple-value results in SystemError: <class 'StopIteration'> returned with exception set #128078

[ZeroIntensity]

You can just use PyErr_ExceptionMatches to skip the PyErr_Occurred call.

[ZeroIntensity]

Do we have any precedent for passing __FILE__ and __LINE__ to errors from C? But anyways, I don't think displaying the exception like this is a good way to do it; we should just set it as a cause. (Also, this should probably be a RuntimeError instead of a SystemError--those should generally not be raisable from Python.)

You could do it with something like this:

PyObject *cause = PyErr_GetRaisedException();
PyErr_SetString(PyExc_RuntimeError, "unexpected exception");
PyObject *err = PyErr_GetRaisedException();
PyException_SetCause(err, cause);
PyErr_SetRaisedException(err);