CI: update flake job software

Update OS to Ubuntu 24.04 LTS, and Python to 3.8 through 3.13

Author: stsnel

.github/workflows/main.yml

@@ -9,11 +9,16 @@ on: [push]
 jobs:
   build:
 
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-24.04
 
     strategy:
       matrix:
-        python-version: [3.6, 3.7, 3.8]
+        python-version: ["3.8",
+                         "3.9",
+                         "3.10",
+                         "3.11",
+                         "3.12",
+                         "3.13"]
 
     steps:
     - uses: actions/checkout@v3