Bump nltk from 3.6.4 to 3.6.6

Bumps [nltk](https://github.com/nltk/nltk) from 3.6.4 to 3.6.6.
- [Release notes](https://github.com/nltk/nltk/releases)
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](nltk/nltk@3.6.4...3.6.6)

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

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

Author: dependabot[bot]

requirements.txt

@@ -1,6 +1,6 @@
 SoundFile==0.10.3.post1
 tensorflow_datasets==4.4.0
-nltk==3.6.4
+nltk==3.6.6
 numpy>=1.19.5
 sentencepiece==0.1.96
 tqdm==4.62.1