[issue8525] Display exceptions' subclasses in help()

[Eric Wieser]

Eric Wieser <wieser.eric+pybug at gmail.com> added the comment:

> I get the following results for builtin objects that have defined subclasses

>From that list, the only unhelpful ones with > 4 items, in my opinion, appear to be `object`, since that just tells you every type that exists, and `tuple`, because that lists every single namedtuple.

> So it is USEFUL to know ALL subclasses of a given Exception class

I agree with this - most of the value here comes from showing the full set of exceptions. If we don't do that, we should probably point the user to calling `cls.__subclasses__()` so they can inspect the full list

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nosy: +Eric.Wieser

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<https://bugs.python.org/issue8525>
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