build(deps): bump nltk from 3.5 to 3.6.5

Bumps [nltk](https://github.com/nltk/nltk) from 3.5 to 3.6.5.
- [Release notes](https://github.com/nltk/nltk/releases)
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](nltk/nltk@3.5...3.6.5)

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

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

requirements-mkdocs.txt

@@ -17,7 +17,7 @@ mkdocs-macros-test==0.1.0
 mkdocs-material==7.0.6
 mkdocs-material-extensions==1.0.1
 mkdocs-minify-plugin==0.3.0
-nltk==3.5
+nltk==3.6.5
 Pygments==2.7.4
 pymdown-extensions==8.1
 python-dateutil==2.8.1