Getting an Authorization Code on Spotify API (POST request)

This is a spotify documentation I'm following. Out of the 3 options of 'Authorization Flows', I'm trying the 'Authorization Code Flow'.

Finished step 1. Have your application request authorization.

Stuck at step 2. Have your application request refresh and access tokens

It's asking to make a POST request that contains the parameters encoded in ´application/x-www-form-urlencoded as defined in the OAuth 2.0 specification:. Here is what I've done so far with my limited knowledge and google search.

import requests
import base64
from html import unescape 

url = "https://accounts.spotify.com/api/token"

params = {
    "grant_type": "authorization_code",
    "code": <authorization code I got from step 1>,
    "redirect_uri": "http://127.0.0.1:5000/", 
}

headers = {
    "user-agent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_5) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/73.0.3683.103 Safari/537.36",    
    "Content-Type": "application/x-www-form-urlencoded",
    "Authorization" : base64.b64encode("{}:{}".format(CLIENT_ID, CLIENT_SECRET).encode('UTF-8')).decode('ascii')
}

html = requests.request('post', url, headers=headers, params=params, data=None) 
print(html.text)

result, with response code 400

{"error":"invalid_client"}

What should I do to make it work? I thought I got all the params right.