CI: pin actions by SHA

This eliminates the possibility of a tag being changed under
us.

Author: tacaswell

.github/workflows/black.yml

@@ -11,4 +11,4 @@ jobs:
       - uses: actions/checkout@v3
         with:
           persist-credentials: false
-      - uses: psf/black@stable
+      - uses: psf/black@8a737e727ac5ab2f1d4cf5876720ed276dc8dc4b # 25.1.0