Bump nltk from 3.6.5 to 3.6.6

Bumps [nltk](https://github.com/nltk/nltk) from 3.6.5 to 3.6.6.
- [Release notes](https://github.com/nltk/nltk/releases)
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](nltk/nltk@3.6.5...3.6.6)

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

Signed-off-by: dependabot[bot] <[email protected]>

requirements.txt

@@ -7,7 +7,7 @@ spacy==2.3.2
 scispacy==0.2.5
 en-ner-bc5cdr-md @ https://s3-us-west-2.amazonaws.com/ai2-s2-scispacy/releases/v0.2.5/en_ner_bc5cdr_md-0.2.5.tar.gz
 scikit-learn==0.23.2
-nltk==3.6.5
+nltk==3.6.6
 pyarrow==1.0.1
 tqdm==4.49.0
 numpy==1.19.2