add exceptiongroup support to retry_if_exception

[jakkdl]

This makes tenacity transparently handle any exceptions wrapped inside an ExceptionGroup, so it'll handle a ValueError() the same as ExceptionGroup("...", [ValueError()]).
If you want it to be more explicit it could be added as a parameter "allow_group".

I have not implemented it for retry_if_exception_cause_type as it gets quite messy conceptually: "retry if any of the causes of any of the raised exceptions, recursively checked both on any exceptiongroup and their containing exceptions, is of one or more types". This doesn't sound like something any sane person would ever want.

I have not implemented retry_if[_not]_exception_message. This one is less weird and I can probably just do it as "retry if the messages of all the exceptions in a group matches" and ignore the exceptiongroup message, but that's still somewhat sketchy.

When reading the tests I also found a random unused _retryable helper. I'm guessing I can just remove it?

Since #480 is not merged I also quickly added py313 to tox & CI

TODO:

1. write changelog
2. write docs

@Zac-HD

[mergify]

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