ci: Adjust images to cover all Python versions

Only Ubuntu 20.04 is still supported with 3.5 and 3.6, so pin to it.
Expand the test to 3.10 and 3.11 at this chance. Quoting is needed now
to avoid that 3.10 becomes 3.1.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

.github/workflows/next.yml

@@ -14,10 +14,10 @@ env:
 jobs:
   perform_tests:
     name: Run tests on kas code
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
-        python-version: [3.5, 3.6, 3.7, 3.8, 3.9]
+        python-version: ["3.5", "3.6", "3.7", "3.8", "3.9", "3.10", "3.11"]
     steps:
       - name: Check out repo
         uses: actions/checkout@v2