Typing on the get_unverified_header function

Hello, thank you for the tool!

It seems the `get_unverified_header` is missing types to use in a strict mypy configuration!

https://github.com/jpadilla/pyjwt/blob/master/jwt/api_jws.py#L202

If I am not mistaken, this might do it:

```
def get_unverified_header(self, jwt: str) -> Dict[str, Any]:
```