Change (or remove) requirements.txt

I'm not sure if `requirements.txt` is needed at all, but if we keep it we should consider changing it according to https://caremad.io/blog/setup-vs-requirement/.

If I understand this correctly, the file should only contain this:

    # See https://caremad.io/blog/setup-vs-requirement/
    
    -e .

I don't know if `pysoundfile` itself would be needed in addition to that ...