ci: Exclude invalid combinations of Python and Django 5.1

Author: simklei

.github/workflows/python-package.yml

@@ -25,12 +25,21 @@ jobs:
           - python-version: 3.9
             django-version: 5.0
 
+          - python-version: 3.9
+            django-version: 5.1
+
           - python-version: 3.8
             django-version: 5.0
 
+          - python-version: 3.8
+            django-version: 5.1
+
           - python-version: 3.7
             django-version: 5.0
 
+          - python-version: 3.7
+            django-version: 5.1
+
           - python-version: 3.7
             django-version: 4.0