update version and citation information

[brittonsmith]

PR Summary

Updating to 3.1.dev0 and citation info to 3.0.

PR Checklist

• [ ] Code passes tests.
• [ ] New features are documented with docstrings and narrative docs.
• [ ] Tests added for fixed bugs or new features.

[manodeep]

Hey @brittonsmith - congrats on the new version release! I notice that you have a CITATION.rst file to display the citation info. Not sure if you saw - last week GitHub released a feature that it would display citation info on the sidebar, if there is a CITATION.cff file present in the repo root. Might be worthwhile considering since that could reduce the burden on the users to cite ytree

Here's one way to generate a citation.cff file from scratch. More info about the cff format is here