show diff if error on black formatting

Author: h-g-s

.github/workflows/test.yml

@@ -24,7 +24,7 @@ jobs:
         run: python -m pip install flake8 black
 
       - name: check style with black
-        run: python -m black mip --line-length=89 --check
+        run: python -m black mip --line-length=89 --check --diff
 
       - name: check for unused imports with flake8
         run: python -m flake8 mip --select=F401 --exclude=__init__.py