Bump pyyaml from 5.1.1 to 5.1.2 (#815)

Bump [pyyaml](https://github.com/yaml/pyyaml) from 5.1.1 to 5.1.2.
- [Release notes](https://github.com/yaml/pyyaml/releases)
- [Changelog](https://github.com/yaml/pyyaml/blob/master/CHANGES)
- [Commits](https://github.com/yaml/pyyaml/commits)

infra/spawn_data_pipeline_requirements.txt

@@ -1,4 +1,4 @@
 taskcluster==15.0.0
 json-e==3.0.0
 requests==2.22.0
-pyyaml==5.1.1
+pyyaml==5.1.2