Use safe yaml loader

[tdomhan]

With this change we make use of the safe yaml loader that only accepts our own defined types and the standard yaml types. Additionally, we make sure to be able load tuples (which are used for buckets and some other parts of the config).

Pull Request Checklist

• Changes are complete (if posting work-in-progress code, prefix your pull request title with '[WIP]'
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• Unit tests pass (pytest)
  - [ ] Were system tests modified? If so did you run these at least 5 times to account for the variation across runs?
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• 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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