ci: drop actions/setup-python, which hangs on this runner

The setup-python action downloads a Python build from GitHub. This runner
cannot reach GitHub reliably for that, and the test job hung on it with no
output. The runner image already carries a usable python3, so the tests use
it directly and install into a local venv.

.gitea/workflows/build_push.yml

@@ -9,14 +9,16 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v4
 
-      - name: Set up Python
-        uses: actions/setup-python@v5
-        with:
-          python-version: '3.11'
+      # NOTE: no actions/setup-python here.  That action downloads a Python
+      # build from GitHub, which this runner cannot reach reliably - the job
+      # hung on it.  The runner image already carries a usable python3, so
+      # the tests use it directly.
+      - name: Show the Python version
+        run: python3 --version
 
       - name: Install dependencies
         run: |
-          python -m venv .venv
+          python3 -m venv .venv
           . .venv/bin/activate
           pip install --upgrade pip
           pip install -r requirements-dev.txt