Use a dedicated token to create the "update constraints" PR.

When updating the Python constraints, create the PR using a dedicated
token instead of the default GITHUB_TOKEN. Using the default token has
the side-effect that the PR will not trigger any workflow, which in
particular means that the test suite (`build-and-test.yml` workflow)
will not be run.

A new token has been created on @gouttegd's account and added to the ODK
repository. This commit amend the "update-constraints" workflow to
specifically use that token.

.github/workflows/constraints.yml

@@ -20,6 +20,7 @@ jobs:
         uses: peter-evans/create-pull-request@v3
         if: ${{ success() }}
         with:
+          token: ${{ secrets.WORKFLOW_TOKEN }}
           commit-message: Update constraints.txt
           title: 'Update constraints.txt'
           body: |