Actions: Add Python 3.10 and remove 3.5.

.github/workflows/tests.yaml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
-        python-version: [3.5, 3.6, 3.7, 3.8, 3.9]
+        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10"]
     steps:
       - name: Start MySQL
         run: |