ExceptionGroup PEP654 proof of concept.

[AndreasBackx]

Type of changes

• [ ] Bug fix
• [x] New feature
• [ ] Documentation / docstrings
• [ ] Tests
• [ ] Other

Checklist

• [x] I've run the latest black with default args on new code.
• [ ] I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• [ ] I've added tests for new code.
• [x] I accept that @willmcgugan may be pedantic in the code review.

Description

This is just a proof of concept for how exception groups may look, primarily looking to get some feedback on:

1. How people would like for it to look.
2. How the implementation should be done primarily, so feedback on the data structure changes and where to put the code.

This implementation does not (yet) care about backwards-compatibility or supportng the exceptiongroup pypi backport, that is to be added later. This is how the current example I've added looks when running,

Compared to the default (which is also printed below it):

This resolves #1859.

[AndreasBackx]

@willmcgugan, when you have some time, could you share some of your critique so I can flesh this out and we can hopefully get this landed? Would like to get some early feedback before going the whole mile.

[AndreasBackx]

@willmcgugan, do you happen to have some time to give this a look?

Marking it as ready for review even though it was my plan to get some initial feedback first. There doesn't seem to be bandwidth and that's okay, though a quick message would be nice on what we could do to get this improved.

[willmcgugan]

Just busy ATM. I will do a pass on Rich soon. Initial impressions are that it is a good idea.