Add numpy to requirements.txt

run.py imports numpy which is not specified in requirements.txt

same with 'six', it tries to import but it isn't present in requirements.txt

This repo is abandoned by the author, I guess.

However, as numpy is already a peer dependency implicitly required by torch, I think adding it to requirements.txt doesn't make a difference. But the case for six, I do think that it needs to be present in requirements.txt.