Nutree

- Add `Tree.build_random_tree()` (experimental).
- Add `GenericNodeData` as wrapper for `dict` data.
- Fixed 7 Tree.from_dict failing to recreate an arbitrary object tree with a mapper.

- BREAKING: Drop Python 3.7 support (EoL 2023-06-27).
- `Tree.save()` accepts a `compression` argument that will enable compression.
`Tree.load()` can detect if the input file has a compression header and will
decompress transparently.
- New traversal methods `LEVEL_ORDER`, `LEVEL_ORDER_RTL`, `ZIGZAG`, `ZIGZAG_RTL`.
- New compact connector styles `'lines32c'`, `'round43c'`, ...
- Save as mermaid flow diagram.

- Support compact serialization forrmat using `key_map` and `value_map`.
- Better support for working with derived classes (overload methods instead of
callbacks).
- Fix invalid UniqueConstraint error message when loading a TypedTree.
- Add optional `tree.print(..., file=IO)` argument.
- Rename `default_connector_style` to `DEFAULT_CONNECTOR_STYLE`

- Implement `Tree(..., shadow_attrs=True)`.
- `tree.load(PATH)` / `tree.save(PATH)` use UTF-8 encoding.
- Add `Tree.system_root` as alias for `Tree._root`.
- Add `Tree.get_toplevel_nodes()` as alias for `tree.children.`
- Support and test with Py3.12: don't forget to update pip, pipenv, and tox.
- Deprecate Python 3.7 support (EoL 2023-06-27).

- BREAKING: renamed `tree.to_dict()` to `tree.to_dict_list()`.
- BREAKING: changed `tree.load()` / `tree.save()` storage format.
- `tree.load()` / `tree.save()` accept path in addition to file objects.