Skip reformatting Darker's code base on old Black

.github/workflows/python-package.yml

@@ -119,9 +119,17 @@ jobs:
         if: matrix.constraints == '--constraint constraints-oldest.txt'
         run: |
           sed -i 's/py311/py39/' pyproject.toml
-      - name: Run Pytest
+      - name: Run Pytest with the Darker plugin on recent Black versions
+        if: matrix.constraints != '--constraint constraints-oldest.txt'
         run: |
           pytest --darker
+      - name: Run Pytest without the Darker plugin on oldest Black version
+        # The reformatting rules used to be a bit different. We don't need to
+        # test reformatting Darker's own code base with old Black versions.
+        # Interoperability is ensured by unit tests.
+        if: matrix.constraints == '--constraint constraints-oldest.txt'
+        run: |
+          pytest
 
   build-sdist-validate-dists:
     runs-on: ubuntu-latest