Address security bug in yaml loading (backport #313)

[mergify]

This is an automatic backport of pull request #313 done by Mergify.

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[aprotyas]

Backporting #313 because invoking view_frames.py from the foxy branch source generates this warning:

/root/ws/install/tf2_tools/lib/tf2_tools/view_frames.py:75: YAMLLoadWarning: calling yaml.load() without
Loader=... is deprecated, as the default Loader is unsafe.
Please read https://msg.pyyaml.org/load for full details.
  data = yaml.load(result.frame_yaml)

[aprotyas]

CI:
Repos file: https://gist.githubusercontent.com/aprotyas/b2f8d5af5ecb15e9b05c25ad4e91ea91/raw/fe0600f807c5a5f3148010bf6c48242a91bce05c/ros2.repos
Build args: --packages-up-to tf2_tools
Test args: --packages-select tf2_tools
Job: https://ci.ros2.org/job/ci_launcher/9466/

• Linux
• Linux-aarch64
• macOS
• Windows

[aprotyas]

Green CI + approval, so I'll merge this PR.