I ran into this issue when making a call to Gitlab CI. Since the above did not work for me (gave me some kind of error about not being able to concatenate bytes to a string), and because the arguments I was attempting to pass were nested, I thought I would post what ended up working for me:

API_ENDPOINT = "https://gitlab.com/api/v4/projects/{}/pipeline".format(GITLAB_PROJECT_ID)

API_TOKEN = "SomeToken"

data = {
    "ref": ref,
    "variables": [
        {
            "key": "ENVIRONMENT",
            "value": some_env
        },
        {   "key": "S3BUCKET",
            "value": some_bucket
        },
    ]
}

req_headers = {
    'Content-Type': 'application/json',
    'PRIVATE-TOKEN': API_TOKEN,
}

http = urllib3.PoolManager()

encoded_data = json.dumps(data).encode('utf-8')

r = http.request('POST', API_ENDPOINT,
             headers=req_headers,
             body=encoded_data)

resp_body = r.data.decode('utf-8')
resp_dict = json.loads(r.data.decode('utf-8'))

logger.info('Response Code: {}'.format(r.status))
logger.info('Response Body: {}'.format(resp_body))

if 'message' in resp_body:
    logfile_msg = 'Failed Gitlab Response-- {} {message}'.format(r.status, **resp_dict)

You have to do slightly more work, and in return have much more control. But I get downvoted in this sub for pointing out that urllib is more powerful than requests, for some reason.