Bump pyyaml from 3.11 to 5.4

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.11 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@3.11...5.4)

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

docs-requirements.txt

@@ -9,7 +9,7 @@ mkdocs==0.15.3
 mkdocs-bootstrap==0.1.1
 mkdocs-bootswatch==0.4.0
 mkdocs-material==0.2.4
-PyYAML==3.11
+PyYAML==5.4
 singledispatch==3.4.0.3
 six==1.10.0
 tornado==4.4.1