Libtree

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* [FIX] Subclasses of type 'str' are now accepted as IDs

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* [FEATURE] Custom UUID4s can be passed to Node.insert_child()

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* [BREAKING] Require PostgreSQL 9.5
* [BREAKING] Replace integer IDs with UUID4 strings

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* [BREAKING] Native Python exceptions have been replaced by custom ones
* [FEATURE] Add 'has_children' property for Node objects
* [FIX] Accessing the 'parent' property of a root object now returns None