Trying to retrieve all contacts in zendesk sell using API

Hello - 

I am trying to retrieve all contacts in zendesk sell using the following code:

import requests
import json

response = requests.get(
    url='https://api.getbase.com/v2/contacts',
    headers={
      'Accept': 'application/json',
      'Authorization': 'Bearer $ACCESS_TOKEN'
    },
    verify=True
  )

print(response.text)

I am getting this output:

{"errors":[{"error":{"code":"unauthorized","message":"request is unauthorized","details":"Required access token is missing, malformed, expired, or invalid."},"meta":{"type":"error","links":{"more_info":"https://developers.getbase.com/docs/rest/articles/errors"}}}],"meta":{"type":"errors","http_status":"401 Unauthorized","logref":"f0e9da8e6f52da411e187017dead7b69","links":{"more_info":"https://developers.getbase.com/docs/rest/articles/errors"}}}

Does anyone know what I am doing wrong here?