Use ExceptionGroups when they become available

Python added [ExceptionGroups](https://docs.python.org/3/library/exceptions.html#exception-groups) in version 3.11. These would be useful in the context the multiprocessing routines used by fab. We should consider using these when 3.10 goes out of support.

Start by reading Python's ExceptionGroups documentation and locating the multiprocessing routines used by fab. Confirm the project's Python 3.10 support status before deciding the scope. Done means an agreed implementation uses ExceptionGroups when that support boundary is reached, with relevant behavior covered by tests.

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