List concat with different types causes error

When I run mypy on this code:

```
from typing import List, Any

def f(a: List[int], b: List[str]):
    return a+b

f([1,2,3], ["3","4","5"])
```

I get this error:
    /home/georgeatmit/type1.py: note: In function "f":
    /home/georgeatmit/type1.py:3: error: Unsupported operand types for + ("list" and List[str])

Is combining lists of different types supported at the moment?
