iscoroutinefunction should use TypeIs

One of the things I find myself writing a lot is

```python
if iscoroutinefunction(myfunction):
    do_something_with_async(myfunction)
else:
    do_something_with_just_sync(myfunction)
```

however the else block doesn't narrow the type to non-awaitable. Shouldn't it? Some of the other `is...` functions in inspect seem to use TypeIs as well.