ci: test on Python 3.7 and 3.9, drop long dead Python 3.6

ubuntu-latest image on GH action no longer has Python 3.6...

.github/workflows/test-suite.yml

@@ -8,7 +8,7 @@ jobs:
     strategy:
       max-parallel: 5
       matrix:
-        python: [3.6, 3.9]
+        python: [3.7, 3.9]
 
     steps:
     - uses: actions/checkout@v3