Transition from `CompositeError` to builtin `ExceptionGroup`

[PEP-654](https://peps.python.org/pep-0654/) adds a new builtin `ExceptionGroup` type to Python, and `except*` syntax for cleanly handling multiple errors. [The backport](https://pypi.org/project/exceptiongroup/) 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. https://github.com/python-trio/trio/pull/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.