Fix CI: use Python 3.12 to match development environment

Python 3.10's unittest.mock._dot_lookup resolves dotted patch paths
differently from 3.12: it walks the path with getattr, finds the
imported Click Command object at 'th_cli.commands.abort_testing' (due
to 'from .abort_testing import abort_testing' in __init__.py), and
then fails trying to get 'get_client' off the Command object.

Python 3.12 correctly resolves the patch target via sys.modules,
finding the module rather than the imported name.

All tests pass locally on 3.12; switching CI to match eliminates
the AttributeError failures across all command modules.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: rquidute

.github/workflows/python-tests.yml

@@ -15,7 +15,7 @@ jobs:
       - name: Set up Python
         uses: actions/setup-python@v5
         with:
-          python-version: "3.10"
+          python-version: "3.12"
 
       - name: Install Poetry
         uses: snok/install-poetry@v1