Deprecate `asyncio.iscoroutinefunction` in favor of `inspect.iscoroutinefunction`.Co-authored-by: Kumar Aditya <kumaraditya@python.org>

Python 3.14 will deprecate asyncio.iscoroutinefunction:python/cpython#122875inspect.iscoroutinefunction exists since 3.5 and our baselineis 3.8, so we can just use it unconditionally.Using a wrapper with @asyncio.coroutine in __get__ wasn'tnecessary (the future from asyncio.ensure_future is awaitable,and the wrapper doesn't do anything asynchronous), so thelogic can be simplified to just call asyncio.ensure_future(to schedule the task and store the result when it'savailable).

`asyncio.iscoroutinefunction` was deprecated inpython/cpython#122875.