Load safely YAML in Python

Summary:
Per https://github.com/yaml/pyyaml/wiki/PyYAML-yaml.load(input)-Deprecation

All our YAML documents can be loaded with the SafeLoader,
as we don't construct Pickle-style Python objects.

Test Plan: utils/next-uid.py

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2114