Specify yaml loader when loading mkdoc toc (#487)

The following warning was thrown on CI build:

    ./scripts/validate_mkdocs.py:40: YAMLLoadWarning: calling
    yaml.load_all() without Loader=... is deprecated, as the default
    Loader is unsafe. Please read https://msg.pyyaml.org/load for full
    details.

Following the deprecation guide, this update simply specifies a
Loader= parameter when loading the mkdoc toc.

scripts/validate_mkdocs.py

@@ -34,7 +34,7 @@ def parse_page(errors_detected, page):
         return errors_detected
 
 stream = open("docs/mkdocs/mkdocs.yml", 'r')
-mkdocs = yaml.load_all(stream)
+mkdocs = yaml.load_all(stream, Loader=yaml.SafeLoader)
 errors_detected = False
 
 for doc in mkdocs: