no debian packages released on focal

[tfoote]

The new python3 target with focal targets was added to stdeb.cfg 7 months ago in #816, but there has been no release since then so there's no package available on focal from debian packages. This means that the prerelease test instructions don't work by default on Focal. http://wiki.ros.org/regression_tests#How_do_I_setup_my_system_to_run_a_prerelease.3F

The quick workaround is to go from pip, but it would be better to cut a new release for users convenience and to make sure that they don't end up with a pip version overlaying.