Update GitHub workflow test matrix:

- Python 3.6 is not available any more (ubuntu-20.04 has been retired)
- Python 3.7 is not available any more on latest ubuntu
- Add Python 3.13 and 3.14 (beta)

Author: RKrahl

.github/workflows/run-tests.yaml

@@ -11,16 +11,17 @@ jobs:
     strategy:
       matrix:
         python-version:
-          - '3.7'
           - '3.8'
           - '3.9'
           - '3.10'
           - '3.11'
           - '3.12'
+          - '3.13'
+          - '3.14.0-beta - 3.14.0'
         os: [ubuntu-latest]
         include:
-          - python-version: '3.6'
-            os: ubuntu-20.04
+          - python-version: '3.7'
+            os: ubuntu-22.04
     steps:
       - name: Check out repository code
         uses: actions/checkout@v4