loongson/pypi: links for nltk

INFO: Because this project isn't in the mirror_whitelist, no releases from root/pypi are included.

loongson/pypi nltk-3.7.tar.gz
loongson/pypi nltk-3.7-py3-none-any.whl
loongson/pypi nltk-3.5-py3-none-any.whl