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

@@ -40,7 +40,7 @@ pymdown-extensions==6.3
 pyparsing==2.4.6
 python-dateutil==2.8.1
 pytz==2019.3
-PyYAML==5.3
+PyYAML==5.4
 pyzmq==18.1.1
 requests==2.22.0
 six==1.14.0