ci: test Python 3.10 in coverage jobs

.github/workflows/ci.yml

@@ -267,6 +267,9 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v2
+      - uses: actions/setup-python@v2
+        with:
+          python-version: "3.10"
       - uses: actions/cache@v2
         with:
           path: |