Bump pyyaml from 5.1 to 5.2

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 5.1 to 5.2.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@5.1...5.2)

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

Author: dependabot[bot]

requirements.txt

@@ -3,7 +3,7 @@ chardet==3.0.4
 idna==2.8
 python-frontmatter==0.5.0
 python-slugify==4.0.0
-PyYAML==5.1
+PyYAML==5.2
 requests==2.22.0
 six==1.13.0
 text-unidecode==1.3