Add support for inspect.iscoroutinefunction() for Coroutine provider

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This improves Python 3.12+ support.

Background:
@asyncio.coroutine is no longer available since 3.11, asyncio.iscoroutinefunction() was a helper function to detect generator-based coroutines, i.e. functions marked with @asyncio.coroutine decorator, which set _is_coroutine attribute. This implementation detail was historically abused to mark sync functions returning awaitables.
Since 3.12, inspect.markcoroutinefunction() is a preferred way to mark sync functions returning awaitables. It is intended to be used with inspect.iscoroutinefunction(). Given the fact that we're in Cython realm, we have to resort to less graceful solution and go deeper by abusing implementation details, again.

Note:
asyncio.iscoroutinefunction() will be deprecated in 3.16, so old marker is still in place for better compatibility.