The execd processor expects that testing.py command to run as a daemon. If the process has already died or completed then this is one reason for that error.
I don’t have access to your script so the first question: is it set up to run as a daemon or not? If you run it manually does it exit quickly? or keep running?
Most probably you get your error, as the python script throws some error. Try to execute the python code outside the context of telegraf and you will see.