chore(deps): bump pyyaml from 5.1 to 5.4 in /requirements

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)

---
updated-dependencies:
- dependency-name: pyyaml
  dependency-type: direct:production
...

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

Author: dependabot[bot]

requirements/base.txt

@@ -13,5 +13,5 @@ sphinx-notfound-page==0.8.3
 beautifulsoup4==4.11.1
 markdownify==0.11.2
 python-slugify==6.1.2
-pyyaml==5.1
+pyyaml==5.4
 frontmatter==3.0.7