[python][ci] Run black linter in the CI (only passes if no black would not change anything)

.gitlab-ci/default.yml

@@ -3,6 +3,7 @@ default:
 
 
 stages:
+  - linting
   - configure
   - select
   - build
@@ -30,6 +31,14 @@ configure:
       - build-cmake
     expire_in: 3 hours
 
+python linting:
+  stage: linting
+  image: registry.gitlab.com/pipeline-components/black:latest
+  script:
+  # only check the python folder for now (Python code related to bindings)
+  # TODO: maybe extend this to the utility scripts?
+    - black --check --verbose -- python
+
 
 select tests:
   stage: select

python/README.md

@@ -69,3 +69,14 @@ You can run all currently existing DuMu<sup>x</sup> Python tests with
 cd dumux/build-cmake
 ctest -L python
 ```
+##  Development
+
+All Python files should be linted by the tool [`black`](https://pypi.org/project/black/).
+You can install `black` with `pip install black` and run it from the dumux top-directory
+
+```
+black ./python
+```
+
+You can also run it on a specific file (replace `./python` by file name)
+This will automatically format the Python files. Run black before every commit changing Python files.