Incorrect check on string interpolation

When sending this type-correct code

``` python
d = {"greeting": "hello", "count": 10}
print("%(greeting)s World. %(count)d%% tests failing" % d)
```

through mypy, I get the following:

```
error: String interpolation mixes specifier with and without mapping keys
```

Which should be accepted; the only interpolator I'm using withot keys is "%%" which is just an escape and can not cause problems
