union in string annotation invalid error

```py
a: "list[int] | None"
```
```py
> mypy test.py --tb --python-version 3.8
test.py:1:4: error: "list" is not subscriptable, use "typing.List" instead [misc]
a: "list[int] | None"
^
Found 1 error in 1 file (checked 1 source file)
```

Reproduce the issue using the shown test.py annotation and the `mypy test.py --tb --python-version 3.8` command. Trace the handling of string annotations and the reported error, then verify that this annotation no longer produces the invalid “list is not subscriptable” diagnostic.

Written by the indexing model from the issue text.