I rewrote my PR 13515:

* threading.excepthook() now gets a single argument which has multiple attributes: (exc_type, exc_value, exc_traceback, thread)

* The default threading.excepthook() implementation in written in C which reduces the risk of missing symbol during Python shutdown. There is also a simple implementation in Python, for other Python implementations which don't want to implement _thread._excepthook() in C.

* New _make_invoke_excepthook() function which handles the gory details for daemon threads. It creates a "local namespace" with references to all required functons and creates a _invoke_excepthook() function.

* _invoke_excepthook() packs arguments as a C structseq / Python namedtuple (depending on the implementation), calls threading.excepthook(). If threading.excepthook() raises an exception, sys.excepthook() is called to handle it.

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First I also added a "stderr" argument to the arguments passed to threading.excepthook(): sys.stderr, or an old copy of sys.stderr if sys.stderr no longer exists or is set to None. But I decided to keep this as an implementation detail instead.