Value of type "stat_result" should be indexable

Hello,

The following code snippet works perfectly:

```python
import stat
import os

os.stat('/tmp')[stat.ST_MODE]
```

But mypy returns an error:

```
$ mypy bug.py 
bug.py:4: error: Value of type "stat_result" is not indexable
```

I believe that stat_result should be indexable.