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

@@ -6,7 +6,7 @@ livereload==2.6.1
 Markdown==3.2.1
 MarkupSafe==1.1.1
 mkdocs==1.1
-PyYAML==5.3
+PyYAML==5.4
 singledispatch==3.4.0.3
 six==1.14.0
 tornado==6.0.4