ci: add a black check job

.gitlab-ci.yml

@@ -11,6 +11,7 @@
     file:
       - "templates/no-duplicated-ci-pipelines.yml"  # use workflow to avoid duplicated pipelines
       - "templates/lint/flake8.yml"  # will do the equivalent of 'tox -e flake8'
+      - "templates/lint/black.yml"  # will do the equivalent of 'tox -e black'
       - "templates/lint/check-manifest.yml"  # will do the equivalent of 'tox -e check-manifest'
       - "templates/lint/yamllint.yml"  # will do the equivalent of 'tox -e yamllint'
       - "templates/tests/py3.yml"  # will do the equivalent of 'tox -e py3'