Update Python versions for testing (MetOffice#392)

* Update Python versions for CI testing

We were still testing with 3.7 and 3.8 which have gone end-of-line. We were also not testing with modern supported versions.
Also specified which branches to run CI testing on.

Author: MatthewHambley

.github/workflows/build.yml

@@ -1,6 +1,14 @@
 name: Build
 
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - main
+      - v*.*
+  pull_request:
+    branches:
+      - main
+      - v*.*
 
 jobs:
   build:
@@ -9,7 +17,7 @@ jobs:
     strategy:
       max-parallel: 4
       matrix:
-        python-version: ['3.7', '3.8', '3.9', '3.10']
+        python-version: ['3.9', '3.10', '3.11', '3.12', '3.13']
 
     steps:
       # Should this step use a cache?