collections.OrderedDict.copy() -> Dict?

mypy 0.4.4

```
note: In function "f":
5: error: Incompatible return value type (got Dict[Any, Any], expected OrderedDict[Any, Any])
```

``` py
import collections

def f(x: collections.OrderedDict) -> collections.OrderedDict:
    y = x.copy()
    return y
```
