update workflow

Author: mjfield2

.github/workflows/tests.yml

@@ -30,7 +30,7 @@ jobs:
       - name: Run tests
         run: pytest --cov=gstatsim --cov-report=html
       - name: Upload coverage report
-        uses: actions/upload-artifact@v3
+        uses: actions/upload-artifact@v4
         with:
           name: coverage-report
-          path: ./htmlcov/
+          path: ./htmlcov/