CI: Clean up EOL Python versions

Github no longer offers Python 2.x at all, so stop running CI on that.

Github no longer offers Python 3.5 or 3.6 on its latest Ubuntu images,
so use an older Ubuntu image to run those.

Signed-off-by: Simon McVittie <smcv@debian.org>

.github/workflows/testing.yml

@@ -26,9 +26,13 @@ jobs:
     strategy:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
-        python-version: [2.7, 3.5, 3.6, 3.7, 3.8, 3.9, '3.10', '3.11', '3.12']
+        python-version: [3.7, 3.8, 3.9, '3.10', '3.11', '3.12']
         no-coverage: [0]
         include:
+          - os: ubuntu-20.04
+            python-version: '3.5'
+          - os: ubuntu-20.04
+            python-version: '3.6'
           - os: ubuntu-latest
             python-version: pypy-2.7
             no-coverage: 1