Switch Python 2.7 and 3.6 to older base images

These are no longer supported on ubuntu-latest, but they work on 20.04.

Author: pganssle

Diff for: .github/workflows/tests.yml

@@ -10,6 +10,16 @@ jobs:
       matrix:
         python-version: ["2.7", "3.6", "3.7", "3.8", "3.9", "3.10", "3.11"]
         os: ["ubuntu-latest", "windows-latest", "macos-latest"]
+        exclude:
+          - python-version: "2.7"
+            os: "ubuntu-latest"
+          - python-version: "3.6"
+            os: "ubuntu-latest"
+        include:
+          - python-version: "2.7"
+            os: "ubuntu-20.04"
+          - python-version: "3.6"
+            os: "ubuntu-20.04"
     env:
       TOXENV: py