threading: fix ExceptHookArgs being a function instead of a type

[bluetech]

The previous typing meant threading.ExceptHookArgs could not be used to type a value.

The new typing follows what cpython does in the happy path (_thread exists rather than the pure-python fallback being used), a bit different than @srittau's suggestion in the issue.

Also, ExceptHookArgs is a PyStructSequence rather than an actual namedtuple, so stubtest complained about it. I changed it to tuple+properties instead. I can also add n_fields, n_sequence_fields, n_unnamed_fields properties which some other structseqs in typeshed use, but seems like obscure internal detail to me so I omitted those.

Fixes #4767.

[srittau]

Thanks!

[jakebailey]

Just saw this change, but I think the fix here changed the visibility of excepthook and ExceptHookArgs due to the clarification made in PEP 484 (#4586, python/peps#1630) to explicitly state that the from ... import X as X form just have X be the same in both places. This is something I believe we already adjusted for in pyright, but I'm not sure that it's been adopted fully in mypy.

Did this fix depend on no longer using the assignment, or was that just a cleanup on top of the other changes?

[bluetech]

Thanks, I didn't know that both Xs need to be the same now. I'll send a PR tomorrow to fix this.

[hauntsaninja]

Thanks for catching, @jakebailey ... Would you mind submitting a PR to fix? (fwiw I adjusted this in mypy in python/mypy#9515 although I'm not sure whether that's been released yet)

edit: looks like my comment submission raced against bluetech's, thanks for offering to fix! :-)

[jakebailey]

Thanks for that link. I can submit a PR later if @bluetech doesn't beat me to it.