Bump pyyaml from 5.1 to 5.4

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 5.1 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...5.4)

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

Author: dependabot[bot]

docs_requirements.txt

@@ -1,5 +1,5 @@
 Jinja2==2.11.3
-PyYAML==5.1
+PyYAML==5.4
 Pygments==2.7.4
 Sphinx==1.2b1
 cov-core==1.7

requirements.txt

@@ -7,5 +7,5 @@ pbr==4.3.0
 pluggy==0.7.1
 py==1.10.0
 pytest==3.8.2
-PyYAML==5.1
+PyYAML==5.4
 six==1.11.0