Experiment with `ExceptionGroup` and `BaseException.add_note()` (in Python 3.11)

[Zac-HD]

PEP 654 introduces a new ExceptionGroup type, which is designed to replace Trio's MultiError and our very own MultipleFailures types. There's also neat new except* syntax for handling them, but that's not our problem. Based on our conversation, Irit Katriel also added BaseException.__note__ so that we can attach our Failing example: strings (etc) to the exception objects, instead of printing them by hand.

So before Python 3.11 is released, preferably in the alphas (i.e. before May 2022), we should get this working - preferably shipped; but at least prototyped to a level that any problems can be fixed and re-tested before 3.11rc1.

Presumably at some point there will be a backports.exceptiongroup package that we can standardize on along with Trio, Pytest, and so on; but until then I'm happy to keep using our own exceptions.

[Zac-HD]

Since PEP-678 has been accepted and implemented (to be released in 3.11.0b1, in early May) and there's an excellent exceptiongroup backport on PyPI, we'll soon be able to jump directly to using the new approach at all times. Concretely, the plan is to:

• Draft everything out using PEP 678: new API for add_note() agronholm/exceptiongroup#4
• Use (Base)ExceptionGroup and .add_note() on all Python versions (via the backport)
• Release both the backport patch and Hypothesis-using-ExceptionGroup-and-add_note shortly after Python beta1

Early-draft branch is at master...Zac-HD:use-exceptiongroup, including some notes on use of .add_note().

So we'll have experience with everything early enough that we shouldn't need further patches, and still have the beta period to shake out any problems if they do arise.