Adding --diff flag to black formatting in CI (#131)

Author: arnasbr

.github/workflows/ci.yml

@@ -47,7 +47,7 @@ jobs:
           flake8  --max-line-length=127
       - name: Formatting check with black
         run: |
-          black . --check
+          black . --check --diff
       - name: Type check with mypy
         run: |
           mypy .