cannot register when using type hintings

If I add type hints to `get` or `post` etc like,

``` python
class API(FlaskView):
  def get(self) -> str:
    return "OK"
```

 it gives the following error.

```
ValueError: Function has keyword-only arguments or annotations, use getfullargspec() API which can support them
```

Looks like you `getfullargspec` is needed insted of `getargspec`.
