Bump up runtime version to python 3.13 (#13313)

* Update runtime version to python 313 in app.yaml

* Update main.py

* Remove new region tag

* Restore original region tag

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Co-authored-by: Katie McLaughlin <[email protected]>

Author: sharmaharisam

appengine/standard_python3/building-an-app/building-an-app-1/app.yaml

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-runtime: python39
+runtime: python313
 
 handlers:
   # This configures Google App Engine to serve the files in the app's static