Bump nltk from 3.3 to 3.4.5

Bumps [nltk](https://github.com/nltk/nltk) from 3.3 to 3.4.5.
- [Release notes](https://github.com/nltk/nltk/releases)
- [Changelog](https://github.com/nltk/nltk/blob/develop/ChangeLog)
- [Commits](nltk/nltk@3.3...3.4.5)

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

Author: dependabot[bot]

requirements.txt

@@ -12,6 +12,6 @@ GitPython==2.1.11
 grpc==0.3-19
 imageio==2.3.0
 jupyter==1.0.0
-nltk==3.3
+nltk==3.4.5
 skimage==0.0
 scikit_learn==0.19.2