Spotify API {'error': 'invalid_client'} Authorization Code Flow [400]

This is one of my many attempts at making a POST request to https://accounts.spotify.com/api/token.

Scope was set to 'playlist-modify-public, playlist-modify-private'.

I'm using Python 3.7, Django 2.1.3.

No matter what I do, response_data returns {'error': 'invalid_client'}

I've tried many things, including passing the client_id/client_secret inside the body of the request as per the official Spotify documentation for this particular request... to no avail.

Please help!

def callback(request):

    auth_token = request.GET.get('code')     # from the URL after user has clicked accept
    code_payload = {
        'grant_type': 'authorization_code',
        'code': str(auth_token),
        'redirect_uri': REDIRECT_URI,
    }

    auth_str = '{}:{}'.format(CLIENT_ID, CLIENT_SECRET)
    b64_auth_str = base64.b64encode(auth_str.encode()).decode()

    headers = {
        'Content-Type': 'application/x-www-form-urlencoded',
        'Authorization': 'Basic {}'.format(b64_auth_str)
    }

    post_request = requests.post(SPOTIFY_TOKEN_URL, data=code_payload, headers=headers)

    response_data = json.loads(post_request.text)
        # ==> {'error': 'invalid_client'}

from Spotify API {'error': 'invalid_client'} Authorization Code Flow [400]