Unable to get Catalyst API access_token

I am trying to get a working example by following the instructions here:

https://docs.catalyst.zoho.com/en/api/oauth2/register-new-client/#RegisteraNewClient

I registered a new "Self Client" and got client_id and client_secret. Then I generated a grant_token as described there.

I then attempted to get the access_token and refresh_tokens as described. I am using Python to send the POST and GET requests. Here is my code.

import requests 
url = 'https://accounts.zoho.com/oauth/v2/token'
data = {
    'code': grant_token,
    'client_id': client_id,
    'client_secret': client_secret,
    'grant_type': 'authorization_code'
}
resp = requests.post(url, json=data)

But it didn't work. I then tried the following code and I get an {'error': 'invalid_client'} response.

import urllib
import jsonparams = f'code={grant_token}' \
         f'&client_id={client_id}' \
         f'&client_secret={client_secret}' \
         f'&grant_type=authorization_code'
bin_params = params.encode('utf-8')
req = urllib.request.Request(f'{url}?', data=bin_params)
response = urllib.request.urlopen(req)
result = json.loads(response.read())

I am stuck. How can I get this to work?