Bumps [nltk](https://github.com/nltk/nltk) from 3.5 to 3.6.6.
- [Release notes](https://github.com/nltk/nltk/releases)
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](https://github.com/nltk/nltk/compare/3.5...3.6.6

)

---
updated-dependencies:
- dependency-name: nltk
  dependency-type: direct:production
...

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