The most recent stable branch is 3.9. We should add an explicit 3.9 recipe. If you copy gnuradio-master.lwr to a new gnuradio39.lwr file and change "master" to "maint-3.9", it should work.
You can also edit the .pybombs/config.yml in the directory for your 3.9 build (not $HOME/.pybombs/config.yml) to override the git branch, something like:
3.9 is fairly close to master, just missing things that would change the API, features under test, etc.
Changing an OOT that works with 3.9 to work with 3.10 should be straightforward when the time comes. No big changes like we had from 3.8 to 3.9.