Pin the tests to run on ubuntu-2204

 version to work with setup-python@v4

ubuntu-latest points to ubuntu-24.04. Some python versions were removed
or not included at the moment. An error like this can be found whilst
running in GH without this change:
Version 3.7 was not found in the local cache
  Error: The version '3.7' with architecture 'x64' was not found for Ubuntu 24.04.

Also adding 3.12 to the mix.

.github/workflows/tests.yml

@@ -67,7 +67,7 @@ jobs:
         tox -e dep
 
   unit_tests:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
 
     strategy:
       fail-fast: false
@@ -79,6 +79,7 @@ jobs:
         - '3.9'
         - '3.10'
         - '3.11'
+        - '3.12'
 
     steps:
     - name: Checkout the source code
@@ -101,7 +102,7 @@ jobs:
         tox -m test
 
   static_type_checking:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-22.04
 
     strategy:
       fail-fast: false
@@ -113,6 +114,7 @@ jobs:
         - '3.9'
         - '3.10'
         - '3.11'
+        - '3.12'
 
     steps:
       - name: Checkout the source code