Littletree

- Add `NetworkXSerializer` for conversion to and from [networkx](https://networkx.org/).
- Faster implementation of `__eq__` for deep trees.
- Two trees can now be equal (`==`) if their roots have a different identifier.
- Replace argument `str_factory` by `formatter` in `tree.show()` and `tree.to_string`.
If passed a string, it will be applied as `formatter.format(node=node)`.

Fix bug in `tree.compare` method

- Improve DotExporter
- Rename `name_factory` to `node_name`.
- Derive file type from file extension if possible. Fall back on png.
- Add `graph_attributes`.
- Make attribute handling more intelligent. Distinguish static and dynamic properties.
- Add parameter `directed` (default: True).
- Remove whitespace around arrows for compacter file size.
- Add MermaidExporter. Similar to DotExporter, but supported by [GitHub](https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/creating-diagrams).
- Make `tree.children` writable for more compatibility with anytree/bigtree
- Add parameters `backend` and `node_label` to `tree.to_image()`

- Remove `tree.show(img=True)`. Use `tree.to_image().show()` instead.
- New method `tree.transform()` that creates a modified or pruned copy of a tree.
- New method `tree.compare()` that compares different trees to each other.
- Rewrite `copy()` in terms of transform.
Use `tree.copy()` for shallow copy.
Use `tree.copy({})` for deep copy.
- Remove style `minimal`, but add style `list` for `tree.show()`.
- Add option `escape_comments` to newick serializer to escape newick modifiers in node data.
- For now use `distance` and `comment` instead of `distance_field` and `comment_field` in NewickSerializer.
- Changed default format of `to_dict` to contain `identifier`.
- Rewrite RowSerializer
- Refactor RelationSerializer

- Streamline `DotExporter`.
Create temporary files in memory.
Add option `as_bytes` to `to_image()`.
Remove unused parameter `separator`.
- Add more default styles to `StringExporter`.
Some have been renamed. `const` has been renamed to `square`.
- Simplify signature of `from_newick`. It accepts a string.
For files a Path or file object should be passed.
- Add `tree.show()` for quickly rendering a tree to console.
Same as `print(tree.to_string())`.

- Make dictserialization work on large trees.