Bump nltk from 3.8.1 to 3.9.4

Bumps [nltk](https://github.com/nltk/nltk) from 3.8.1 to 3.9.4.
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](nltk/nltk@3.8.1...3.9.4)

---
updated-dependencies:
- dependency-name: nltk
  dependency-version: 3.9.4
  dependency-type: indirect
...

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Author: dependabot[bot]