Allow the use of types.ModuleType

Based on this [SO question](http://stackoverflow.com/questions/39827307/mypy-class-module-annotation), I tried this code:

```
import types
def f() -> types.ModuleType:
    return types
```

and it failed type check with ` Incompatible return value type (got "module", expected "ModuleType")`. I think names exported in `types.*` are appropriate for cases like this?