Union types for dict keys are not inferred in union context

mypy: 0.650
Python: 3.7.1

I'm not sure if this is a bug or a new feature, however the following code is unexpected (in my mind, especially coming from TypeScript):

```python
d = {"1": 2, 3: 4}
reveal_type(d)  # builtins.dict[builtins.object*, builtins.int*]
```

I would have expected mypy to infer the type of `d` as `builtins.dict[Union[builtins.str*, builtins.int*], builtins.int*]`.