Use weights_only=True when loading SockeyeModel parameters.

[mjdenkowski]

Use weights_only=True when loading SockeyeModel parameters. From torch.load documentation:

torch.load() unless weights_only parameter is set to True, uses pickle module implicitly, which is known to be insecure.

Pull Request Checklist

• Changes are complete (if posting work-in-progress code, prefix your pull request title with '[WIP]'
  until you can check this box.
• Unit tests pass (pytest)
• System tests pass (pytest test/system)
• Passed code style checking (./style-check.sh)
• Updated major/minor version in sockeye/__init__.py. Major version bump if this is a backwards incompatible change.
• Updated CHANGELOG.md

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[mjdenkowski]

It looks like macos-latest no longer supports Python versions 3.7 and 3.8. I've set our checks to use macos-12, which supports both.