Fix `python-version` in the GitHub Actions

Current `ubuntu-latest` (Ubuntu 22.04 for x86_64) provides the following
versions of Python:
 - 3.10.8
 - 3.11.0
 - 3.7.15
 - 3.8.15
 - 3.9.15

.github/workflows/main.yml

@@ -15,8 +15,8 @@ jobs:
       max-parallel: 4
       fail-fast: false
       matrix:
-        python-version: [3.6, 3.7, 3.8, 3.9]
-        # github runner with python 3.5 currently fails to install this package
+        # Current ubuntu-latest (Ubuntu 22.04) provides the following versions of Python:
+        python-version: [3.7, 3.8, 3.9, 3.10, 3.11]
 
     steps: