It looks like travis is using the travis.yml from the branch, but runs the test script from the branch merged into master. This causes pull requests created before the QA tests python 3 transition to fail due to missing python3-zmq package.
This happens in, for example, #8006.
This temporarily reverts fa05e22e919b7e2e816606f0c0d3dea1bd325bfd (#7851). It can be restored when this is no longer an issue.