CI: add --diff to black

@ReinhardPrix : This should make a failed job report back what are the actual formatting issues. This would be useful as it seems that black is getting stricter in higher python versions, e.g. the changes here passed black 19.10b0 for me on CIT (python 3.6.8) but fail what the CI is running.

Do you have any concerns that the larger job logs might be a problem?

.gitlab-ci.yml

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