Bump nltk from 3.4.5 to 3.6.6

Bumps [nltk](https://github.com/nltk/nltk) from 3.4.5 to 3.6.6.
- [Release notes](https://github.com/nltk/nltk/releases)
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](nltk/nltk@3.4.5...3.6.6)

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

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

requirements.txt

@@ -1,4 +1,4 @@
-nltk==3.4.5
+nltk==3.6.6
 JPype1==1.1.2
 snowballstemmer==1.2.1
 nlptoolkit-MorphologicalAnalysis==1.0.44