Pin to python 3.12 in upb/macos tests.

Python 3.13 seems to have some issues with our dependencies and both numpy and setuptools fails to install.

PiperOrigin-RevId: 689955916

.github/workflows/test_upb.yml

@@ -118,6 +118,7 @@ jobs:
       - name: "Setup Python"
         uses: actions/setup-python@61a6322f88396a6271a6ee3565807d608ecaddd1 # v4.7.0
         with:
+          python-version: 3.12
           cache: pip
           cache-dependency-path: 'python/requirements.txt'
       - name: Run tests