Encontrei estes passos online e eles fizeram o truque:
$ sudo apt-get install python3-pip
$ sudo pip3 install pyyaml
$ wget http://www.nltk.org/nltk3-alpha/nltk-3.0a3.tar.gz
$ tar -xzvf nltk-3.0a3.tar.gz
$ cd nltk-3.0a3/
$ sudo python3 setup.py install
$ python3
>>> import nltk
>>> from nltk.corpus import brown
>>> print(brown.sents()[0])
['The', 'Fulton', 'County', 'Grand', 'Jury', 'said', 'Friday', 'an', 'investigation', 'of', "Atlanta's", 'recent', 'primary', 'election', 'produced', '''', 'no', 'evidence',
"''", 'that', 'any', 'irregularities', 'took', 'place', '.']