GH-73991: Prune pathlib.Path.copy() and copy_into() arguments

[barneygale]

Remove ignore and on_error arguments from pathlib.Path.copy[_into](), because these arguments are under-designed. Specifically:

• ignore is appropriated from shutil.copytree(), but it's not clear how it should apply when the user copies a non-directory. We've changed the callback signature from the shutil version, but I'm not confident the new signature is as good as it can be.
• on_error is a generalisation of shutil.copytree()'s error handling, which is to accumulate exceptions and raise a single shutil.Error at the end. It's not obvious which solution is better.

Additionally, these arguments may be challenging to implement in future user subclasses of PathBase, which might utilise a native recursive copying method.

No news because copy() and copy_into() are unreleased.

• Issue: Support moving across filesystems in pathlib.Path, as shutil.move() does #73991

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[pfmoore]

I do think we need some replacement for the removed arguments, but unlike delete, I think it's more about enabling additional use cases and less about making sure that basic usage is OK. Specifically, I think ignore is important for filtered copies (for example, copy a project directory but don't copy generated files like *.pyc or *.obj). I'm less sure about on_error - mostly it's the same arguments as for delete, but in this case I don't know of any commonly-encountered cases that would need to be supported in day to day use.

So +1 from me on doing this as a temporary limitation, to get a minimal working implementation released.

[barneygale]

Thanks very much for the review! I agree with everything you wrote heh. I'll look to add support for filtering and customizing error handling soon. We might get a user request for them before too long!