ci(gitlab-ci): use templates from a common repository

.gitlab-ci-extended.yml

+---
+# follow this doc to extend the already existing jobs:
+# https://docs.gitlab.com/ee/ci/yaml/includes.html#overriding-external-template-values
+
+# as a reminder the existing jobs are located here:
+# https://forge.extranet.logilab.fr/open-source/gitlab-templates/
+
+# or simply add more jobs here if you need them (use the already existing stages)
+
+# /!\ remember to uncomment the include in the .gitlab-ci.yml /!\

.gitlab-ci.yml

@@ -3,6 +3,9 @@ default:
   image: python:3.7
 
 include:
+  # uncomment and uses to customize/extend the configuration here if needed
+  # (it needs to be at the same level than "- project")
+  # - ".gitlab-ci-extended.yml"
   - project: "open-source/gitlab-ci-templates"
     ref: "branch/default"
     file:

MANIFEST.in

@@ -13,3 +13,4 @@ exclude cubicweb-blog.spec
 exclude .gitlab-ci.yml
 exclude .yamllint
 exclude .cube-doctor.yml
+exclude .gitlab-ci-extended.yml