Use new version of codecov upload action

Author: PicoCentauri

.github/workflows/tests.yml

@@ -27,6 +27,10 @@ jobs:
 
     - name: run Python tests
       run: tox -e tests
-    - uses: codecov/codecov-action@v3
+
+    - name: upload to codecov.io
+      uses: codecov/codecov-action@v4
       with:
+        fail_ci_if_error: true
         files: ./tests/coverage.xml
+        token: ${{ secrets.CODECOV_TOKEN }}