fix aws-py-stepfunctions (pulumi#1596)

AWS no longer supports lambdas with the python3.7 runtime. Update it to
the latest runtime to get this example to work again.

I noticed this was failing in the tests currently, and the error message
suggests upgrading to the latest python.

aws-py-stepfunctions/__main__.py

@@ -6,7 +6,7 @@
 
 hello_world_fn = lambda_.Function('helloWorldFunction',
  role=iam.lambda_role.arn,
- runtime="python3.7",
+ runtime="python3.12",
  handler="hello.handler",
  code=pulumi.AssetArchive({
  '.': pulumi.FileArchive('./step_hello')