Bump nltk from 3.5 to 3.8.1 in /src/Backend

Bumps [nltk](https://github.com/nltk/nltk) from 3.5 to 3.8.1.
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](nltk/nltk@3.5...3.8.1)

---
updated-dependencies:
- dependency-name: nltk
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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