Delete can't contain data

Some APIs require to send data through the HTTP `delete` request. The `slumber` doesn't allow this, so I have to use this hack:

```python
        resp = self.api.lists(self.list_id).unsubscribe._request(
            "DELETE",
            data={"email": email},
        )
        if not 200 <= resp.status_code <= 299:
            raise Exception("Not unsubscribed")
```