Aha, now that's interesting.

The isolated venv was then probably the reason why pip didn't see
python3-sphinx in https://travis-ci.org/enkore/i3pystatus/builds/38247577

.travis.yml

@@ -1,9 +1,12 @@
 language: python
 python:
   - "3.3"
-install:
+virtualenv:
+  system_site_packages: true
+before_install:
   - "sudo apt-get install -qq python3-sphinx"
-  - "pip3 install -r .travis-requirements.txt"
+install:
+  - "pip3 install -r dev-requirements.txt"
 script:
   - "pep8  --ignore E501 i3pystatus tests"
   - "PYTHONPATH=. py.test"        # run unit tests