ensure ci for 3.6 runs on older ubuntu version that still supports 3.6 installation

Signed-off-by: oleg.hoefling <[email protected]>

Author: hoefling

.github/workflows/tests.yml

@@ -8,12 +8,14 @@ on:
 
 jobs:
   build:
-
-    runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9, '3.10', '3.11']
-
+        os: ubuntu-latest
+        python-version: [3.7, 3.8, 3.9, '3.10', '3.11']
+        include:
+          - os: ubuntu-20.04
+            python-version: 3.6
+    runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}