Bump pyyaml from 5.1.2 to 5.4

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 5.1.2 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@5.1.2...5.4)

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

Author: dependabot[bot]

requirements.txt

@@ -58,7 +58,7 @@ pyparsing==2.4.0
 pyrsistent==0.15.4
 python-dateutil==2.8.0
 pytz==2019.1
-PyYAML==5.1.2
+PyYAML==5.4
 pyzmq==18.0.2
 requests==2.22.0
 retrying==1.3.3