bpo-46752: Introduce task groups in asyncio by gvanrossum · Pull Request #31270 · python/cpython

My plan is roughly the following:

- [x] Copy the files from EdgeDb without modifications
      (named following asyncio conventions)
- [ ] Bang on the tests until they will run
- [ ] Bang on the tests until they pass
- [ ] Remove pre-3.11 compatibility code
- [ ] Switch from MultiError to ExceptionGroup
- [ ] Other cleanup
- [ ] Add a public API to tasks.py to replace `__cancel_requested__`

Two remaining issues:

- [ ] _test_taskgroup_21 doesn't work (it didn't in EdgeDb either)
- [ ] the test framework complains about a change to the event loop policy

This required some changes to the tests since EdgeDb's MultiError
has some ergonomic conveniences that ExceptionGroup doesn't:

- A helper method to get the types of the exceptions
- It puts the number and types of the exceptions in the message

Also, in one case (test_taskgroup_14) an EG nested inside another
EG was raised, whereas the original code just raised one EG.
This remains to be investigated.