fix: use app token to push to gh-pages branch

Author: mattwang44

.github/workflows/ci.yml

@@ -28,11 +28,17 @@ jobs:
       - name: Build HTML Docs
         run: VERSION=${{ github.event.repository.default_branch }} JOBS=4 MODE=html make all
 
+      - uses: actions/create-github-app-token@v2
+        id: app-token
+        with:
+          app-id: ${{ secrets.APP_ID }}
+          private-key: ${{ secrets.APP_PRIVATE_KEY }}
+
       - name: Deploy PR Doc Preview
         uses: rossjrw/pr-preview-action@v1
         with:
           source-dir: ../cpython/Doc/build/html
-          token: ${{ secrets.GITHUB_TOKEN }}
+          token: ${{ steps.app-token.outputs.token }}
           preview-branch: gh-pages
           umbrella-dir: pr-preview
           action: auto