Transition from CompositeError to builtin ExceptionGroup

[Zac-HD]

PEP-654 adds a new builtin ExceptionGroup type to Python, and except* syntax for cleanly handling multiple errors. The backport is already widely used, including by Pytest, Trio, Hypothesis, and many others. I'd love to see ipyparallel join in, so that all our users get interoperable tooling and - once they're on recent versions of Python - nice syntax too.

Logistically, this is a substantial lift, but following e.g. python-trio/trio#2213 will hopefully be a lot easier than doing the whole thing from scratch. You might even choose to wait a while for the ecosystem to mature, but I thought it was worth opening an issue now - if nothing else, it's relevant to any other proposed changes to CompositeError - and I'm confident that switching over will be the best way forward within the next few years.

[minrk]

I haven't had time to look deeply into it, but that sounds perfect! Not sure when I'll get to it, but it seems very sensible.