This is not where Jython 3 is developing.
The latest version of Jython remains 2.7.x (see Jython for that). A Jython 3.x should appear there, but exploratory work towards it is not stable at the time of writing. Please watch the jython-dev mailing list for developements.
This repo represents an attempt made in 2016 to implement a Jython 3.5, that is, with language and runtime compatibility with CPython 3.5, along with continued substantial support of the Python ecosystem. It is a sandbox for that venture. This code having branched from an early 2.7, the architecture is that of Jython 2, without taking advantage of the (then fairly new) dynamic language support. It has not been updated with subsequent bug-fixes made on Jython 2.7, and there is no plan to to do so.
Please see ACKNOWLEDGMENTS for details about Jython's copyright, license, contributors, and mailing lists; and NEWS for detailed release notes, including bugs fixed, backwards breaking changes, and new features.