run-tests.yml: Drop EOL Python 3.8 and add Python 3.13

Author: hartwork

.github/workflows/run-tests.yml

@@ -19,7 +19,7 @@ jobs:
     name: Run the test suite
     strategy:
       matrix:
-        python-version: [3.8, 3.12]  # no particular need for in-between versions
+        python-version: [3.9, 3.13]  # no particular need for in-between versions
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683  # v4.2.2