'None' is marked as unknown variable when used as return type

With Python extension 2018.9.2 on VSCode 1.28.2 on OS X 10.14:

```python
def f() -> None:
    pass
```

```
{
	"owner": "_generated_diagnostic_collection_name_#1",
	"code": "used-before-assignment",
	"severity": 4,
	"message": "unknown variable 'None'",
	"source": "Python (analysis)",
	"startLineNumber": 1,
	"startColumn": 12,
	"endLineNumber": 1,
	"endColumn": 16
}
```