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