[issue45292] Implement PEP 654

[Irit Katriel]

New submission from Irit Katriel <iritkatriel at gmail.com>:

We will implement Exception Groups and except* in a series of PRs:

1. Add the ExceptionGroup and BaseExceptionGroup classes
2. Update traceback rendering code (python and C) for exception groups.
3. Implement except*
4. Write documentation sections.

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue45292>
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