Pin black to 22.12.0 to avoid spurious style changes (#35282)

We need to sync changes on major versions with
spackbot, to avoid the bot saying everything is
allright and have CI failing nonetheless.

.github/workflows/valid-style.yml

@@ -44,7 +44,7 @@ jobs:
         cache: 'pip'
     - name: Install Python packages
       run: |
-        python3 -m pip install --upgrade pip six setuptools types-six black mypy isort clingo flake8
+        python3 -m pip install --upgrade pip six setuptools types-six black==22.12.0 mypy isort clingo flake8
     - name: Setup git configuration
       run: |
         # Need this for the git tests to succeed.