API create-semantic-model error ""Parts: Must be a non-empty collection with no null elements""

I think I've figured it out - it's annoying.
The python requests.post() has two possible inputs (well, three if you include params). The usual one is 'data' and it expects a dict. The other is 'json', which also expects a dict-like structure, but coerces it into application/json.
(note I tried data = json.dumps(payload) and got an invalid media error)

spencer_sa So the issue was in the Content-Type Header,Right?

It should be 

Content-Type: application/json

 and the correct usage should be like that:

response = requests.post(
    url,
    json=payload_dict,      # <-- correct
    headers={
        "Authorization": f"Bearer {token}",
        "Content-Type": "application/json"
    }
)