Bump nltk from 3.8.1 to 3.9

Bumps [nltk](https://github.com/nltk/nltk) from 3.8.1 to 3.9.
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](nltk/nltk@3.8.1...3.9)

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

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

requirements.txt

@@ -60,7 +60,7 @@ marshmallow==2.16.3
 marshmallow-polyfield==3.1
 marshmallow-sqlalchemy==0.10.0
 mock==1.3.0
-nltk==3.8.1
+nltk==3.9
 nose==1.3.7
 odfpy==1.4.1
 openpyxl==3.0.3