"HTTP Error 429: Too Many Requests" when using extend_dataset.py

Hi,

I'm interested in using your extend_dataset.py script, but keep getting the same error: 
```urllib.error.HTTPError: HTTP Error 429: Too Many Requests```
I'm trying the script on a dummy *.csv containing 100 English sentences, with all settings in their default.  Any thoughts on how to circumvent this problem?

Thanks a lot!
