add variable for optional_dependency_tag

python/quality/package_black/README.md

+[![Black Logo](https://raw.githubusercontent.com/psf/black/main/docs/_static/logo2-readme.png)](https://black.readthedocs.io/en/stable/)
+
 ## Objective
 
 Run [Black](https://github.com/psf/black) on your Python code to ensure it complies with the coding
@@ -9,6 +11,12 @@ style guide.
 2. Include template in your CI/CD conf (see quick use above)
 3. Customize the template if it's relevant
 
+## Variables
+
+| Name                      | Description                                                                                            | Default |
+|---------------------------|--------------------------------------------------------------------------------------------------------|---------|
+| `OPTIONAL_DEPENDENCY_TAG` | If `black` is listed as a main dependency, leave variable empty, otherwise fill it with `[<TAGNAME>]`. | `""` |
+
 ## Add <a href="https://github.com/psf/black"><img alt="Code style: black" src="https://img.shields.io/badge/code%20style-black-000000.svg"></a> cartouche to your project README.md
 ```markdown
 <a href="https://github.com/psf/black">

python/quality/package_black/package_black.yml

@@ -5,7 +5,7 @@ black:
         - python --version  # Debug
         - python -m venv venv --upgrade-deps || python -m venv venv
         - source venv/bin/activate
-        - pip install .[QUALITY]
+        - pip install .[OPTIONAL_DEPENDENCY_TAG]
         - package_black . --check --diff --quiet
     only:
         changes:
@@ -13,3 +13,5 @@ black:
             - tests/**/*.py
             - .gitlab-ci.yml
             - pyproject.toml
+    variables:
+        OPTIONAL_DEPENDENCY_TAG: ""
\ No newline at end of file