github actions: limit python versions to >= 3.9

Author: gabrielfalcao

.github/workflows/pythonapp.yml

@@ -13,9 +13,10 @@ jobs:
     strategy:
       matrix:
         python:
-          - "3.6"
-          - "3.7"
-          - "3.8"
+          # # it appears that sphinx 8.2.1 requires python version >= 3.9
+          # - "3.6"
+          # - "3.7"
+          # - "3.8"
           - "3.9"
           - "3.10"
           - "3.11"