Configure codecov token for code coverage

Author: jlaine

.github/workflows/tests.yml

@@ -83,8 +83,10 @@ jobs:
           coverage xml
         shell: bash
       - name: Upload coverage report
-        uses: codecov/codecov-action@v4
         if: matrix.python != 'pypy3'
+        uses: codecov/codecov-action@v4
+        with:
+          token: ${{ secrets.CODECOV_TOKEN }}
 
   package-source:
     runs-on: ubuntu-latest