add (Base)ExceptionGroup

[JelleZijlstra]

Fixes #6632

[srittau]

One question below, but LGTM.

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[agronholm]

A couple questions:

1. Why is the second argument to ExceptionGroup named differently than that of BaseExceptionGroup? This definition makes it look like it could be passed as a keyword argument, which it can't be.
2. Why is the return type of the exceptions property typed as a Sequence when it always returns a tuple, regardless of the type of the initial exceptions sequence?

I'm happy to submit a follow-up PR to fix these.

[JelleZijlstra]

@agronholm good catch about the exceptions argument. They should both be positional-only.

I think I used Sequence because the PEP (https://www.python.org/dev/peps/pep-0654/#exceptiongroup-and-baseexceptiongroup) says it's a sequence, but you're right that we prefer concrete types like tuple in typeshed.

[agronholm]

The PEP doesn't really define the exceptions property but it is mentioned there. The constructor argument __exceptions is correct here, as it can be any sequence, but the implementation turns it into a tuple in the getter of exceptions.

[JelleZijlstra]

Yes, the summary is that you're right on both counts :) Feel free to submit a PR!

[agronholm]

Done: #6895

[agronholm]

I noticed another issue: BaseExceptionGroup and ExceptionGroup are actually generic, but this is not reflected in the stubs. Thankfully tuple is covariant so there is no conflict in the exceptions property. I'll send another PR.