invoices.get_csv_by_uuid fails with JSONDecodeError

Hello DigitalOcean,

```
---------------------------------------------------------------------------
JSONDecodeError Traceback (most recent call last)
[](https://localhost:8080/#) in ()
24 print(latest_uuid)
25
---> 26 invoice_resp = client.invoices.get_csv_by_uuid(latest_uuid)
27 invoice_resp

5 frames
[/usr/lib/python3.11/json/decoder.py](https://localhost:8080/#) in raw_decode(self, s, idx)
353 obj, end = self.scan_once(s, idx)
354 except StopIteration as err:
--> 355 raise JSONDecodeError("Expecting value", s, err.value) from None
356 return obj, end

JSONDecodeError: Expecting value: line 1 column 1 (char 0)
```

I suspect this call fails because the API command returns a string and not JSON.

`get_by_uuid` for the same invoice UUID has no issues, likely because the response is JSON.

Thanks for your consideration of this issue.

Mike