Fix memory leak from retained exception tracebacks#168
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When an async task fails, error._traceback and self._frame hold live
references to frame objects and their locals:
AsyncTask._error → ._traceback → traceback → frame → f_locals
AsyncTask._frame → frame → f_locals, f_globals
GC cannot collect these because they are reachable from the live
AsyncTask (held by callers or creator chains), not unreachable cycles.
Fix: clear self._frame in _computed() after the task finishes, and
clear error._traceback after generator.throw() propagates it. Also
migrate to single-arg throw(error) (3-arg form is deprecated).
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When an async task fails, error._traceback and self._frame hold live references to frame objects and their locals:
GC cannot collect these because they are reachable from the live AsyncTask (held by callers or creator chains), not unreachable cycles.
Fix: clear self._frame in _computed() after the task finishes, and clear error._traceback after generator.throw() propagates it. Also migrate to single-arg throw(error) (3-arg form is deprecated).