Did you see what @Brandon_Peterson pointed out? You are doing a GET request in your postman picture but you're doing a POST in your script.
My guess is you need to do it with a GET (and perhaps use params instead of data). Also, don't convert to json first. Read the docs I sent you -
The function itself encodes the dictionary to JSON. It expects a dictionary as one of the acceptable datatypes. If you give it JSON directly - that may be messing with things.