Puppet uses the non-keyword variant of Psych's YAML.safe_load, which was deprecated in 3.1.0 with the replacement also introduced in that version. Psych 4.0.0 has removed the deprecated variant and thus we should switch to the new variant albeit with a version check since we still support older Ruby versions, which ship pre-3.1.0 Psych versions.
Furthermore, YAML.load_file has been changed to a safe variant in 4.0.1 breaking some tests. Puppet's wrapper for it should be updated to use YAML.unsafe_load_file when available (since 3.3.2) to keep the same behaviour.
As a minor side issue rdoc has switched to Psych 4.0.0 in a minor version causing CI breakage due to rubocop not having been updated for the deprecation yet.