Even Python 2 functions support setting arbitrary attributes, so contextlib2.contextmanager can always just add an __annotations__ dict to decorated functions if it doesn't already exist.

```
>>> def f():
...     pass
... 
>>> f.__annotations__
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'function' object has no attribute '__annotations__'
>>> f.__annotations__ = {}
>>> f.__annotations__
{}
```