ci: update upload-artifact action to v4

The upload-artifact@v2 action was deprecated since a while, and is now
completely disabled (forceful CI failure). Update to v4 to get CI back.

This also requires changing the artifact names, as the v4 action no
longer supports uploading different artifacts with the same name.

Signed-off-by: Mathieu Choplain <[email protected]>

Author: Mathieu Choplain

.github/workflows/test.yml

@@ -31,7 +31,7 @@ jobs:
             --cov-report html \
             -vv
       - name: store coverage report 
-        uses: actions/upload-artifact@v2
+        uses: actions/upload-artifact@v4
         with:
-          name: coverage
+          name: coverage-${{ matrix.python-version }}
           path: htmlcov