Bump pyyaml from 5.2 to 5.4 in /backend

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 5.2 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@5.2...5.4)

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

Author: dependabot[bot]

backend/requirements.txt

@@ -96,7 +96,7 @@ PyPDF2==1.26.0
 pyrsistent==0.15.6
 python-dateutil==2.8.0
 pytz==2019.2
-PyYAML==5.2
+PyYAML==5.4
 pyzmq==19.0.0
 requests==2.20.1
 scikit-learn==0.21.3