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Experimenting with this locally, it looks like the C level hang is indeed due to #25782, but the fact that we're implicitly trying to do "exc.__context__ = exc" is a new bug in _GeneratorContextManager introduced by the PEP 479 changes.
However, I'm now wondering whether that's also revealed a more general oversight in ExitStack's "_fix_exception_context" internal helper function: it's not explicitly handling the case where an __exit__ method re-raises the exception that was passed in, rather than returning a false value (indicating to the context management machinery that it should re-raise the original exception). I just hadn't noticed that before since all stdlib context managers are well-behaved in that regard (modulo bugs like the PEP 479 one).
Some failed attempts to create a simpler reproducer than Victor's last example show it isn't straightforward to get the current code to misbehave, but an upfront check for "new_exc is old_exc" may still be a worthwhile defensive coding measure.