[github-actions] fix python versions (2.7, 3.6) not found for ci

.github/workflows/ci.yml

@@ -12,13 +12,17 @@ on:
 jobs:
   ci:
     name: Test with different versions of Python 🐍
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         version: [2.7, 3.6, 3.11]
+        include:
+          - version: 2.7
+            container: python:2.7.18-buster
     steps:
       - uses: actions/checkout@v3
       - name: Set up Python
+        if: matrix.version != '2.7'
         uses: actions/setup-python@v4
         with:
           python-version: ${{ matrix.version }}