ci: bump actions to Node 24 runtimes and clean up cibuildwheel config

- Bump actions/upload-artifact to v6, actions/setup-python to v6,
  actions/checkout to v5, and docker/setup-qemu-action to v4 so all
  actions run on Node 24 (Node 20 is deprecated on GitHub runners).
- Bump pypa/cibuildwheel to v3.4.1 so its bundled actions/setup-python
  also runs on Node 24.
- Drop CIBW_SKIP=pp*: PyPy is no longer enabled by default in
  cibuildwheel 3.x, so the selector matched nothing and triggered an
  "Invalid skip selector" warning.
- Add CIBW_TEST_SKIP for cp38 macOS arm64 to silence the cibuildwheel
  warning that the arm64 slice of cp38 universal2 wheels can't be tested
  (the cp38 installer is x86_64-only).

.github/workflows/docs.yml

@@ -11,9 +11,9 @@ jobs:
   docs:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v5
     - name: Set up Python
-      uses: actions/setup-python@v5
+      uses: actions/setup-python@v6
       with:
         python-version: '3.12'
     - name: Install dependencies
@@ -28,7 +28,7 @@ jobs:
       run: |
         sphinx-build docs build/html
     - name: Upload documentation artifact
-      uses: actions/upload-artifact@v4
+      uses: actions/upload-artifact@v6
       with:
         name: html-docs
         path: build/html
@@ -40,9 +40,9 @@ jobs:
     permissions:
       contents: write
     steps:
-    - uses: actions/checkout@v4
+    - uses: actions/checkout@v5
     - name: Set up Python
-      uses: actions/setup-python@v5
+      uses: actions/setup-python@v6
       with:
         python-version: '3.12'
     - name: Install dependencies