Bump pyyaml from 3.12 to 5.4

Bumps [pyyaml](https://github.com/yaml/pyyaml) from 3.12 to 5.4.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](yaml/pyyaml@3.12...5.4)

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

requirements_base.txt

@@ -30,7 +30,7 @@ pytest==2.9.2
 pytest-django==3.0.0
 python-redis-lock==3.3.1
 pytz==2017.3
-PyYAML==3.12
+PyYAML==5.4
 raven==5.25.0
 redis==2.10.5
 requests==2.11.1