This release is a prerelease of PyO3 0.21 specifically aimed at confirming the new "Bound" API delivers the expected payoffs to downstream projects and that the migration pathway is manageable.
Some documentation is not yet completely polished, and there are some deprecation warnings which are due to be added. The expectation is that this work will take a week or two at most, and the final 0.21 release will follow at that point.
To read about the new Bound API, best to start with the migration guide entry.
If you test out the new API and have feedback, please open a new GitHub issue or discussion, or reach out on PyO3's new Discord. Feedback in this time period before the final release is particularly appreciated.
A more detailed release announcement and acknowledgements will be written with the final release.