add python 3.13 to build workflow

Author: SvenKlaassen

.github/workflows/pytest.yml

@@ -28,6 +28,7 @@ jobs:
           - {os: 'ubuntu-latest', python-version: '3.10'}
           - {os: 'ubuntu-latest', python-version: '3.11'}
           - {os: 'ubuntu-latest', python-version: '3.12'}
+          - {os: 'ubuntu-latest', python-version: '3.13'}
 
     steps:
     - uses: actions/checkout@v4