The package appears to use py3versions -i to determine the "installed" Python versions.
However, this can cause issues if a Python transition is in progress as the -minimal variant of the previous version (eg. python3.X-minimal) remains installed in many environments. This variant then provides enough of an interpreter to count as being "installed" but not enough for the tests themselves to succeed in most cases. This then prevents the overall transition from taking place.
Please replace this will a call to all "supported" packages instead (eg. py3versions -s and ensure python3-all is listed in the build dependencies.
Refer to https://lists.debian.org/debian-devel/2020/03/msg00280.html for details.