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?

Compare the backport's ContextVar behavior with Python's built-in contextvars using the generic annotation example from the issue. Add regression coverage for ContextVar[str] annotations and verify that the Celery-related usage works as expected; done means the backport accepts the same annotation form.

Written by the indexing model from the issue text.