Add Python 3.11 to supported versions (#16)

Co-authored-by: Philip Hackstock <[email protected]>

Author: danielhuppmann

.github/workflows/pytest.yaml

@@ -31,7 +31,7 @@ jobs:
       #       check-out repo and set-up python
       #----------------------------------------------
       - name: Check out repository
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
       - name: Set up python
         id: setup-python
         uses: actions/setup-python@v4