Consistent sep for subtree

[kayjan]

Description

For https://github.com/kayjan/bigtree/issues/303

Testing

Additional notes

Checklist

I have read through the contributing guidelines and ensured that

• [x] I have added a descriptive title for this pull request.
• [x] I have followed the convention and standards, and my code is checked for style and correctness.
• [x] I have added test cases, and unit tests pass with 100% code coverage.
• [x] I have updated the documentation and code docstrings.

Checklist (for reviewer)

• [x] I have added label (breaking / enhancement / bug / documentation) to this pull request, if applicable.
• [x] I will ensure this change is captured in the CHANGELOG.md file.

[github-actions[bot]]

Package	Line Rate	Branch Rate	Complexity	Health
bigtree	100%	100%	0	✔
bigtree.binarytree	100%	100%	0	✔
bigtree.dag	100%	100%	0	✔
bigtree.node	100%	100%	0	✔
bigtree.tree	100%	100%	0	✔
bigtree.utils	100%	100%	0	✔
Summary	100% (2349 / 2349)	100% (990 / 990)	0	✔