travis: include PyGTK setup

Installing PyGTK requires actual GTK system package. The easiest way for
that is using system site-packages - and this means matching Python
version with the Travis environment (Ubuntu bionic).

.travis.yml

@@ -13,8 +13,12 @@ env:
 
 jobs:
   include:
-    - python: '3.5'
-      install: pip install --quiet -r ci/requirements.txt
+    - python: '3.6'  # needs to match bionic
+      virtualenv:
+        system_site_packages: true
+      install:
+        - sudo apt-get -y install python3-gi gir1.2-gtk-3.0
+        - pip install --quiet -r ci/requirements.txt
       env: TESTS_ONLY=1
       script:
        - ./run-tests