chore: add test for py 3.9

Author: fsbraun

.github/workflows/codecov.yml

@@ -16,7 +16,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        python-version: ["3.10", "3.11", "3.12"]  # latest release minus two
+        python-version: ["3.9", "3.10", "3.11", "3.12"]  # latest release minus two
         requirements-file: [
             dj42_cms311.txt,
             dj42_cms41.txt,
@@ -27,6 +27,10 @@ jobs:
             ubuntu-latest,
         ]
         exclude:
+          - python-version: "3.9"
+            requirements-file: dj50_cms41.txt
+          - python-version: "3.9"
+            requirements-file: dj51_cms41.txt
           - python-version: "3.12"
             requirements-file: dj42_cms311.txt