Use python 3.9.7 for black.

Author: FragmentedPacket

.github/workflows/ci.yml

@@ -13,6 +13,8 @@ jobs:
         uses: "actions/checkout@v2"
       - name: "Setup environment"
         uses: "networktocode/gh-action-setup-poetry-environment@v1"
+        with:
+          python-version: "3.9.7"
       - name: "Linting: black"
         run: "poetry run invoke black"
   bandit: