Nutree

- `t0.diff(t1, ...)` adds nodes from t1 when possible, so the new status is
used for modified nodes.
- `t0.diff(t1, ...)` marks both, source and target nodes, as modified if
applicable.

- Implement check for node modifications in `tree.diff(..., compare=True)`.
- `DictWrapper` supports comparision with `==`.

- BREAKING:
- `kind` parameter is now mandatory for `add()` and related methods.
`kind=None` is still allowed to use the default (inherit or 'child').
- Rename `shadow_attrs` argument to `forward_attrs`.
- Enforce that the same object instance is not added multiple times to one parent.
- Rename `GenericNodeData` to `DictWrapper` and remove support for attribut access.
- Drop support for Python 3.8
- mermaid: change mapper signatures and defaults
- tree.to_rdf() is now available for Tree (not only TypedTree).
- New method `node.up()` allows method chaining when adding nodes.
- Pass pyright 'typeCheckingMode = "standard"'.
- Use generic typing for improved type checking, e.g. use `tree = Tree[Animals]()`
to create a type-aware container.

- 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.