Typing annotations not working as intented

Hi! I've found a difference between the built-in version of contextvars and this backport. 

Generic type annotations are not supported. This works in python:

```
 c: contextvars.ContextVar[str] = contextvars.ContextVar('test')
```

but not in this backport. It caused a pretty surprising  issue with celery https://github.com/celery/celery/issues/6534

Would it be possible to fix it? 