ci: bump checkout/setup-python/upload-artifact to Node-24 majors

GitHub will force Node 24 on JS actions starting 2026-06-02 and remove
the Node 20 runtime on 2026-09-16. Bumping:
  - actions/checkout            v4 -> v6 (node24 since v6.0.0)
  - actions/setup-python        v5 -> v6 (node24 since v6.0.0)
  - actions/upload-artifact     v4 -> v7 (node24 since v6.0.0)

Our upload-artifact use is a single conditional upload (gated by
matrix.python-version == '3.11'), so the v5 'no-overwrite / unique
name per matrix leg' contract is already satisfied -- no further
changes needed. ubuntu-latest is well past runner 2.327.1 minimum.

Author: CrispStrobe

.github/workflows/ci.yml

@@ -15,10 +15,10 @@ jobs:
         python-version: ["3.10", "3.11", "3.12"]
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
 
       - name: Set up Python ${{ matrix.python-version }}
-        uses: actions/setup-python@v5
+        uses: actions/setup-python@v6
         with:
           python-version: ${{ matrix.python-version }}
           cache: pip
@@ -48,7 +48,7 @@ jobs:
 
       - name: Upload coverage XML
         if: matrix.python-version == '3.11'
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v7
         with:
           name: coverage-xml
           path: coverage.xml