Make asyncio task groups prevent child tasks from being GCed

[1st1]

We discovered that sometimes child tasks in a task group can be picked up by a GC which leads to extremely hard to debug problems. This is fixable by switching to use a set() instead of weakset to internally track child tasks. I'll make a PR shortly.

See also geldata/gel#3914.

[AlexWaygood]

Closing, as I believe this was fixed in #93299, and backported in #93305. (Please reopen if there's more to do here).