Merge branch 'black-diff' into 'master'

CI: add --diff to black

See merge request GregAshton/PyFstat!27

.gitlab-ci.yml

@@ -36,5 +36,5 @@ static:
   tags: [ pyFstat ]
   script:
     - source ${VENV_DIR}/bin/activate
-    - black --check .
+    - black --check --diff .
     # - flake8 .  ## not ready