Bump nltk from 3.5 to 3.9

Bumps [nltk](https://github.com/nltk/nltk) from 3.5 to 3.9.
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](nltk/nltk@3.5...3.9)

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

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -1,4 +1,4 @@
-nltk==3.5
+nltk==3.9
 numpy~=1.18.2
 dataclasses>=0.6
 pathlib