Bump pyyaml from 5.1.1 to 5.1.2

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 5.1.1 to 5.1.2.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](https://github.com/yaml/pyyaml/commits)

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

Author: dependabot-preview[bot]

requirements.txt

@@ -8,7 +8,7 @@ livereload==2.6.1
 Markdown==3.1.1
 MarkupSafe==1.1.1
 mkdocs==1.0.4
-PyYAML==5.1.1
+PyYAML==5.1.2
 requests==2.22.0
 singledispatch==3.4.0.3
 six==1.12.0