Replace @asyncio.coroutine with “async def” for Python 3.11

[musicinmybrain]

In the tests, replace the asyncio.coroutine decorator, which is deprecated since Python 3.8 and removed in 3.11, with “async def”.

Because async functionality was not available in Python 2.7, and async def was introduced in 3.5, this does not appear to break the tests, except that flake8 in the Python 2.7 tox environment chokes on a SyntaxError for the two test files that are modified in this PR. I’m not sure what you would prefer to do about that.