Bump pyyaml from 5.3.1 to 5.4 in /requirements

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 5.3.1 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/5.3.1...5.4)

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

requirements/requirements.txt

@@ -18,7 +18,7 @@ drf-yasg==1.20.0
 easy-thumbnails==2.5
 python-decouple==3.1
 psycopg2-binary==2.8.6
-pyyaml==5.3.1
+pyyaml==5.4
 pytz==2019.3
 python-magic==0.4.15
 unipath==1.1