The standard contextmanager decorator produces a wrapper that itself can be used as a decorator

```
@contextmanager
def some_context():
    ...
    yield

@some_context()
def some_function():
    # we are inside a with some_context() now. 
    ...


```

When I created a version of asynccontextmanager internally before it was available in the  stdLib I copied this behavior and I have people internally to facebook using this behavior,  Was there a reason this behavior was not replicated to asynccontextmanager?  

I have a diff an tests to add