Use asyncgen hooks to call aclose()

Closes #1638. I originally had `finalizer()` cause a warning if it failed to run `aclose()`, but this ended up making the tests fairly noisy on Python <= 3.12. Instead, I added a note to the documentation. I don't understand why those same tests did not cause the RuntimeError I saw. I'm worried that there is some subtle issue with the order simulator coroutines are being cleaned up. In the meantime, this fix should at least be an improvement. Let me know if you'd like any changes.