Bump pyyaml from 5.1 to 5.4 in /modules/sync

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](https://github.com/yaml/pyyaml/compare/5.1...5.4)

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

modules/sync/requirements.txt

@@ -5,7 +5,7 @@ Pillow==8.1.1
 pymongo==3.3.0
 PyPDF2==1.26.0
 python-frontmatter==0.3.1
-PyYAML==5.1
+PyYAML==5.4
 requests==2.20.0
 six==1.10.0
 smartcrop==0.1