Bump nltk from 3.2.4 to 3.6.6

Bumps [nltk](https://github.com/nltk/nltk) from 3.2.4 to 3.6.6.
- [Release notes](https://github.com/nltk/nltk/releases)
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](nltk/nltk@3.2.4...3.6.6)

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

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

requirements.txt

@@ -4,6 +4,6 @@ matplotlib==2.2.2
 setuptools==36.2.5
 termcolor==1.1.0
 numpy==1.19.1
-nltk==3.2.4
+nltk==3.6.6
 ujson==1.35
 ftfy==4.4.3