Revert "Try bumping python version to 3.10 so we can get a newer FileCheck"

This reverts commit 658a91d.

Author: ldionne

.github/workflows/tox.yaml

@@ -13,7 +13,7 @@ jobs:
     - name: Set up Python ${{ matrix.python-version }}
       uses: actions/setup-python@v2
       with:
-        python-version: '3.10'
+        python-version: 3.9
     - name: Install dependencies
       run: |
         python -m pip install --upgrade pip