add __copy__ and __deep_copy__ to Cancelled

[jakkdl]

See https://github.com/python-trio/flake8-async/issues/298 for why I want to be able to copy Cancelled.

I suppose there's a minor footgun added if people start copying Cancelled, NoPublicConstructor was added for a reason, but I think the gain outweighs it. Though of course another option is to simply get rid of NoPublicConstructor

I initially tried to write cleaner methods, where I was using __dict__ and the like, but they do not appear to pick up the relevant dunders so I simply listed everything relevant from https://docs.python.org/3/library/exceptions.html#exception-context

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[jakkdl]

This is a far better copy than most exception objects have, FYI. (I left a comment on the flake8-trio issue that motivated this.)

Thank you! I think we don't need to bother doing it either then, and I'll throw out the complicated logic.

[jakkdl]

it appears copy.copy has bigger issues with exceptions than trio.Cancelled, see https://github.com/python-trio/flake8-async/issues/298#issuecomment-2424971988 I'll probably close this PR

[jakkdl]

okay I think my original reasoning for this is no longer valid - copying is too unreliable to use in a general library like trio-websocket that needs to handle arbitrary user exceptions, but we should probably make it possible to copy & pickle Cancelled for general usability reasons.

That's fairly straightforward by implementing __reduce__, but it's maybe bad that all other NoPublicConstructor classes remain unpickleable? I'm at least finding one somewhat related issue: https://github.com/python-trio/trio/issues/2135