conversion script pt to ggml not working

Whisper seem to have switched to a different tokenizer library: [pull request](https://github.com/openai/whisper/pull/1044), and the conversion script [convert-pt-to-ggml.py](https://github.com/ggerganov/whisper.cpp/blob/0a2d1210bcb98978214bbf4e100922a413afd39d/models/convert-pt-to-ggml.py) has stopped working. I think it needs to be updated to use the new .tiktoken files.