Bump pyyaml from 5.3 to 5.4

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 5.3 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@5.3...5.4)

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

Author: dependabot[bot]

requirements.txt

@@ -80,7 +80,7 @@ pyparsing==2.4.7
 pyrsistent==0.17.3
 python-dateutil==2.8.1
 pytz==2019.3
-PyYAML==5.3
+PyYAML==5.4
 pyzmq==20.0.0
 regex==2020.2.20
 requests==2.23.0