python 3.13 not yet supported, drop it

Author: petercorke

.github/workflows/pythonpackage.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        python-version: [3.9, "3.10", 3.11, 3.12, 3.13]
+        python-version: [3.9, "3.10", 3.11, 3.12]
     steps:
     - uses: actions/checkout@v2
     - name: Set up Python ${{ matrix.python-version }}