Bump nltk from 3.6.5 to 3.9

Bumps [nltk](https://github.com/nltk/nltk) from 3.6.5 to 3.9.
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](nltk/nltk@3.6.5...3.9)

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

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

requirements.txt

@@ -196,7 +196,7 @@ nbconvert @ file:///C:/ci/nbconvert_1624479160025/work
 nbformat @ file:///tmp/build/80754af9/nbformat_1617383369282/work
 nest-asyncio @ file:///tmp/build/80754af9/nest-asyncio_1613680548246/work
 networkx @ file:///tmp/build/80754af9/networkx_1633639043937/work
-nltk==3.6.5
+nltk==3.9
 nose @ file:///tmp/build/80754af9/nose_1606773131901/work
 notebook @ file:///C:/ci/notebook_1635393701545/work
 numba @ file:///C:/ci/numba_1635186062888/work