Fix showing SystemExit exception raise inside except handler (#14503) Doing something like this: ```python try: 5 / 0 except Exception as e: raise SystemExit ``` was hitting an error inside UltraTB, creating a long traceback of its internals (which, ironically, UltraTB itself then displays correctly :-). `ListTB.get_exception_only()` calls the `ListTB.structured_traceback()` method *specifically* - even if `self` is a subclass, it won't use the subclass's method. However, the exception chaining in that method uses recursion by calling `self.structured_traceback()`, which will use a subclass's method. Tuples were added as an option there to support exception chaining, but not all of the machinery in connected classes expects a tuple. This just skips the exception chaining logic for the `etb=None` case, when we're showing the exception only. I'm not sure this is necessarily the best fix, but I didn't want to spend too much time following code around a module that's old enough to vote. Closes #12104