Python requests: 403 on /oauth/token with client credentials

Hitting /oauth/token with POST and client credentials. Getting 403 back. Code looks standard.

import requests
r = requests.post('https://api.mypurecloud.com/oauth/token', data={'grant_type': 'client_credentials'}, auth=(client_id, client_secret))
print(r.status_code, r.text)

Checked scopes. All good. What am I missing?