add TaskGroup here or in another library

asyncio could really use a `TaskGroup` implementation, as provided by [Trio](https://trio.readthedocs.io/en/stable/) (aka nurseries), [Curio](https://curio.readthedocs.io/en/stable/), [aiotools](https://aiotools.readthedocs.io/en/latest/aiotools.taskgroup.html), and [EdgeDB](https://github.com/edgedb/edgedb/blob/master/edb/common/taskgroup.py).

`TaskGroup` was originally slated to be added Python as long ago as 3.8, but was held up because exception group handling was not available. However, exception groups are now being added: [PEP 654](https://www.python.org/dev/peps/pep-0654/), [PR](https://github.com/python/cpython/pull/29581)). Once that PEP is implemented, `TaskGroup` will probably follow. But the timeline for PEP 654 to go into the MicroPython core is unclear. So any TaskGroup we add will not be quite compatible, and we probably want to add it in an ancillary library (`adafruit_asyncio` or `asynciox` or `xasyncio` or ??).

Compare the referenced Trio, Curio, aiotools, and EdgeDB implementations, starting with edb/common/taskgroup.py, and review the PEP 654 and linked CPython pull request context. First establish whether TaskGroup belongs in this library or an ancillary library. Done means the placement, compatibility expectations, and implementation scope are agreed.

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