Use codecov action v5

Author: dannyadair

.github/workflows/tests.yaml

@@ -26,10 +26,6 @@ jobs:
       - name: Run tests with pytest and collect coverage
         run: pytest --cov=pdfbaker --cov-report=xml
       - name: Upload coverage to Codecov
-        uses: codecov/codecov-action@v3
+        uses: codecov/codecov-action@v5
         with:
           token: ${{ secrets.CODECOV_TOKEN }}
-          file: coverage.xml
-          name: pytest
-          flags: unittests
-          slug: pythonnz/pdfbaker