fix: use python=3.13 for tests_on_pr

Author: ycexiao

.github/workflows/tests-on-pr.yml

@@ -11,5 +11,6 @@ jobs:
       project: diffpy.apps
       c_extension: false
       headless: false
+      python_version: 3.13
     secrets:
       CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}