gh-154791: Fix asyncio Future losing traceback on repeated result() call

[iaroslavkrasikov]

PR #30274 fixed repeating traceback frames in asyncio.Future by storing tracebacks separately from exceptions in fut_exception_tb. However, it also added Py_CLEAR(fut->fut_exception_tb) in future_get_result() after the first result() call. This means any subsequent await/result() raises with PyException_SetTraceback(exc, None), losing traceback frames.

The _PyFuture implementation never clears _exception_tb and has always behaved correctly. This change makes the C implementation match.

The reference cycle concern motivating the Py_CLEAR is already covered: fut_exception_tb is registered in FutureObj_traverse, so GC can collect any cycle through it.

• Issue: asyncio: initial exception traceback frames are lost for Future.result() #154791

[python-cla-bot]

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