github: Update Python versions as 3.6 is no installed anymore

Also use strings as 3.10 is otherwise interpreted as number and
becomes 3.1.

.github/workflows/ci.yml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
+        python-version: ['3.7', '3.8', '3.9', '3.10', '3.11']
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}