ci: install tox in py38 job

A better approach would be to backport python definition in several jobs inside gitlab-ci-templates

.gitlab-ci.yml

@@ -32,6 +32,8 @@ stages:
 py38:
   extends: py3
   image: python:3.8
+  before_script:
+    - pip install tox
 
 py3:
   image: python:3.7