ci: Upgrade python to 3.13

This is to address the following error on CI:

```
Run actions/setup-python@v4
Installed versions
  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.
  The list of all available versions can be found here: https://raw.githubusercontent.com/actions/python-versions/main/versions-manifest.json
```

.github/workflows/ci.yaml

@@ -33,7 +33,7 @@ jobs:
       - name: Set up python
         uses: actions/setup-python@v4
         with:
-          python-version: 3.7
+          python-version: 3.13
 
       - name: Set up chart-testing
         uses: helm/[email protected]