Confusing error message when using AnyStr

``` python
from typing import AnyStr

def f(x: AnyStr, y: AnyStr) -> AnyStr:
    print(x + y)

f('one', b'two')
```

This correctly fails to typecheck because the second arg is bytes, but the error message is hard to follow:

```
test.py:6: error: Type argument 1 of "f" has incompatible value "object"
```
