Add codecov token

.github/workflows/python-coverage.yml

@@ -9,6 +9,10 @@ on:
 jobs:
   coverage:
     runs-on: ubuntu-latest
+
+    # Not intended for forks.
+    if: github.repository == 'optuna/optuna-dashboard'
+
     steps:
     - name: Checkout
       uses: actions/checkout@v3
@@ -39,4 +43,6 @@ jobs:
     - name: Upload coverage to Codecov
       uses: codecov/codecov-action@v3
       with:
+        token: ${{ secrets.CODECOV_TOKEN }}
         file: ./coverage.xml
+        fail_ci_if_error: true