Pin ubuntu version (#903)

.github/workflows/ci.yml

@@ -61,7 +61,10 @@ jobs:
 
   test:
     name: Test
-    runs-on: ubuntu-latest
+    # Specific ubuntu version to support python 3.6 testing
+    # see https://github.com/actions/setup-python/issues/544#issuecomment-1332535877 for details
+    # move to ubuntu-latest when we drop 3.6
+    runs-on: ubuntu-20.04
     strategy:
       matrix:
         python-version: