Actions: Drop Python 3.6 and add 3.11-dev (#542)

Author: methane

.github/workflows/tests.yaml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
-        python-version: ["3.6", "3.7", "3.8", "3.9", "3.10"]
+        python-version: ["3.7", "3.8", "3.9", "3.10", "3.11-dev"]
     steps:
       - name: Start MySQL
         run: |