fix: adding JSON stringify to example python function

We need a JSON string for the body in order to access it in our response message. See this comment: aws-amplify/amplify-js#6390 (comment)

I had this issue with my own function which was returning null on my JSON.stringify(response). I found the above comment and it made it work! i.e. I could access the return body.

packages/amplify-e2e-tests/src/__tests__/function_3.test.ts

@@ -52,7 +52,7 @@ describe('python function tests', () => {
     'Access-Control-Allow-Origin': '*',
     'Access-Control-Allow-Methods': 'OPTIONS,POST,GET',
   };
-  const message: string = 'Hello from your new Amplify Python lambda!';
+  const message: string = JSON.stringify('Hello from your new Amplify Python lambda!');
   const helloWorldSuccessOutput = {
     statusCode: statusCode,
     headers: headers,

packages/amplify-python-function-template-provider/resources/hello-world/index.py

@@ -1,12 +1,15 @@
+import json
+
 def handler(event, context):
   print('received event:')
   print(event)
+
   return {
       'statusCode': 200,
       'headers': {
           'Access-Control-Allow-Headers': '*',
           'Access-Control-Allow-Origin': '*',
           'Access-Control-Allow-Methods': 'OPTIONS,POST,GET'
       },
-      'body': "Hello from your new Amplify Python lambda!"
+      'body': json.dumps('Hello from your new Amplify Python lambda!')
   }