test: gitlab-ci needs pdflatex binary

.gitlab-ci.yml

-image: python
+image: python:latest
 
 stages:
   - tests
@@ -8,10 +8,18 @@ before_script:
   - pip install tox
 
 py3:
+  before_script:
+    - apt-get update
+    - apt-get install texlive-latex-recommended -y
+    - pip install tox
   stage: tests
   script: tox -e py3
 
 py3-from-forge:
+  before_script:
+    - apt-get update
+    - apt-get install texlive-latex-recommended -y
+    - pip install tox
   stage: tests
   script: tox -e py3-from-forge