dev-util/gtk-doc build does not support python 3.8 or 3.9, so it's one of the few packages that require python 3.7. Reproducible: Always Steps to Reproduce: Attempt to upgrade all packages to use python_single_target_python3_8 or python_single_targets_python3_9, and remove python 3.6 and 3.7. Actual Results: dev-util/gtk-doc requires at most python 3.7. Expected Results: All packages are rebuilt to use python 3.8 or 3.9.
*** This bug has been marked as a duplicate of bug 747502 ***