Bump actions/upload-artifact from 5 to 6

Bumps [actions/upload-artifact](https://github.com/actions/upload-artifact) from 5 to 6.
- [Release notes](https://github.com/actions/upload-artifact/releases)
- [Commits](actions/upload-artifact@v5...v6)

---
updated-dependencies:
- dependency-name: actions/upload-artifact
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/tests.yaml

@@ -98,7 +98,7 @@ jobs:
 
     - name: Store coverage file
       if: ${{ !cancelled() && !matrix.skip-coverage }}
-      uses: actions/upload-artifact@v5
+      uses: actions/upload-artifact@v6
       with:
         name: coverage-${{ matrix.python-version }}-${{ matrix.tox-env }}
         path: .coverage-job-*
@@ -153,7 +153,7 @@ jobs:
 
     - name: Upload HTML report on failure
       if: ${{ failure() }}
-      uses: actions/upload-artifact@v5
+      uses: actions/upload-artifact@v6
       with:
         name: html-report
         path: htmlcov