tools/bisector: Fix ruamel usage of DEFAULT_MAPPING_TAG
FIX
Recent versions of ruamel.yaml changed DEFAULT_MAPPING_TAG from a string to a new non-hashable object. Ensure we use the tag name to avoid any exception.
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FIX
Recent versions of ruamel.yaml changed DEFAULT_MAPPING_TAG from a string to a new non-hashable object. Ensure we use the tag name to avoid any exception.