ensure pre-commit runs with a particular Python version

.github/workflows/test.yml

@@ -68,7 +68,9 @@ jobs:
     runs-on: ubuntu-20.04
     steps:
     - uses: actions/checkout@v1
-    - uses: actions/setup-python@v1
+    - uses: actions/setup-python@v2
+      with:
+        python-version: 3.9
     - name: set PY
       run: echo >>$GITHUB_ENV PY="$(python -c 'import hashlib, sys;print(hashlib.sha256(sys.version.encode()+sys.executable.encode()).hexdigest())')"
     - uses: actions/cache@v1