Bump nltk from 3.5 to 3.8.1

Bumps [nltk](https://github.com/nltk/nltk) from 3.5 to 3.8.1.
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](nltk/nltk@3.5...3.8.1)

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

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

requirements.txt

@@ -1,7 +1,7 @@
 # Automatically generated by https://github.com/damnever/pigar.
 
 matplotlib == 3.2.2
-nltk == 3.5
+nltk == 3.8.1
 pandas == 1.0.5
 pandas_datareader == 0.9.0
 tweepy == 3.10.0