Specifically enable Xcode 16.4; and add debug handler.

Author: freakboy3742

.github/workflows/ci.yaml

@@ -205,6 +205,15 @@ jobs:
         path: dist
         merge-multiple: true
 
+    - name: Set up Xcode
+      # GitHub recommends explicitly selecting the desired Xcode version:
+      # https://github.com/actions/runner-images/issues/12541#issuecomment-3083850140
+      # This became a necessity as a result of
+      # https://github.com/actions/runner-images/issues/12541 and
+      # https://github.com/actions/runner-images/issues/12751.
+      run: |
+        sudo xcode-select --switch /Applications/Xcode_16.4.app
+
     - name: Set up Python
       uses: actions/[email protected]
       with:
@@ -233,6 +242,14 @@ jobs:
         # - test_ctypes as a test of FFI
         python -m testbed run --verbose ${{ matrix.testbed-args }} -- test --single-process --rerun -W test_builtin test_grammar test_os test_bz2 test_ctypes
 
+    # This step is only needed if you're trying to diagnose test failures that
+    # only occur in CI, and can't be reproduced locally. When it runs, it will
+    # open an SSH server (URL reported in the logs) so you can ssh into the CI
+    # machine.
+    - name: Setup tmate session
+      uses: mxschmitt/action-tmate@v3
+      if: failure()
+
   crossenv-test:
     name: Cross-platform env test (${{ matrix.multiarch }})
     runs-on: macOS-latest