[CI] Only run python3.8 tests on 3.8 CI job

.gitlab-ci.yml

@@ -23,7 +23,7 @@ test:
   before_script:
     - adduser --disabled-password --gecos "Test User" testuser
   script:
-    - sudo -u testuser tox
+    - sudo -u testuser tox -e py38
   artifacts:
     paths:
       - htmlcov/