Pin style CI job to Python 3.9

.github/workflows/test.yml

@@ -54,6 +54,8 @@ jobs:
     steps:
     - uses: actions/checkout@v2
     - uses: actions/setup-python@v2
+      with:
+        python-version: '3.9'
     - uses: pre-commit/action@v2.0.0
 
   test: