`dmypy suggest` isn't working for return types

```py
def f(a): # dmypy suggest: (str) -> None
return 1

a: int = f("")
```

Start with the Python reproducer in the issue and run `dmypy suggest` against it. Trace how the command handles the `dmypy suggest: (str) -> None` annotation and return type, then add or update a focused regression test. Done means the suggestion is handled correctly for this example and the test passes.

Written by the indexing model from the issue text.