Travis APT addon feature has been used to install
- djvulibre-bin
- graphviz
- liblua5.1-dev
Travis has not been executing the APT addon for the jobs in the `matrix: include:` section after 8004cdfe29 due to https://github.com/travis-ci/travis-ci/issues/5405 . See [[https://travis-ci.org/wikimedia/pywikibot-core/jobs/78144119|build #2754.9]] for the first job log without the APT packages section at the top, compared to [[https://travis-ci.org/wikimedia/pywikibot-core/builds/78087106|build #2753]]
i.e. the following are not the same
```
os: linux
```
```
os:
- linux
```
The `apt:` addon is inherited in `matrix: include:` in the former, but not in the latter.
The unit tests automatically skip tests, and this went unnoticed.
At least djvu tests downgraded to skip some tests.
graphviz and liblua5.1-dev are not used by all of these jobs, as the relevant tests depend on python modules which are only installed with `PYSETUP_TEST_EXTRAS=1` (or `requirements.txt`, but that came later)