Bump pyyaml from 3.11 to 5.4 in /requirements

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.11 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/3.11...5.4)

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

requirements/requirements.txt

@@ -15,7 +15,7 @@ Pillow==2.9.0
 pisa==3.0.33
 psycopg2==2.6.1
 python-memcached==1.53
-PyYAML==3.11
+PyYAML==5.4
 reportlab==2.7
 requests==2.8.1
 six==1.10.0