In tests, use iscoroutinefunction from inspect rather than asyncio

[musicinmybrain]

Summary

In the tests, this replaces asyncio.iscoroutinefunction with inspect.iscoroutinefunction. The asyncio version is deprecated and slated for removal in Python 3.16. The inspect version was introduced in Python 3.5.

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change. No new tests were needed; the change is in test code.
• I've updated the documentation accordingly. No updates were required.