ci: remove quotes from python version to fix ci

Author: zmx27

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

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