Twitter API returned 401 (unauthorized), invalid or expired token

Question

I am using the code below to publish Tweet. This is a fairly standard authentication procedure, but for some reason I cannot authenticate. The error I get is thisTwitter API returned a 401 (Unauthorized), Invalid or expired token.

from twython import Twython, TwythonError
import requests

APP_KEY = 'rpOzpgp2FZNJqsq0' #fake key
APP_SECRET = 'FKBJWXOJwXTblhi1xBl4PtKgPemNFvumH' #fake secret

twitter = Twython(APP_KEY, APP_SECRET)
auth = twitter.get_authentication_tokens()

OAUTH_TOKEN = auth['oauth_token']
OAUTH_TOKEN_SECRET = auth['oauth_token_secret']

twitter = Twython(APP_KEY, APP_SECRET, OAUTH_TOKEN, OAUTH_TOKEN_SECRET)

oauth_verifier_url = auth['auth_url']
oauth_verifier = requests.get(oauth_verifier_url)

# Getting the FINAL authentication tokens
final_step = twitter.get_authorized_tokens(oauth_verifier)

OAUTH_TOKEN = final_step['oauth_token']
OAUTH_TOKEN_SECRET = auth['oauth_token_secret']

twitter = Twython(APP_KEY, APP_SECRET, OAUTH_TOKEN, OAUTH_TOKEN_SECRET)

twitter.update_status(status='Yo')

Here is the complete error message:

Traceback (most recent call last):
  File "test.py", line 20, in <module>
    final_step = twitter.get_authorized_tokens(oauth_verifier)
  File "/Library/Frameworks/Python.framework/Versions/3.4/lib/python3.4/site-packages/twython/api.py", line 380, in get_authorized_tokens
    ken'), error_code=response.status_code)
twython.exceptions.TwythonError: Twitter API returned a 401 (Unauthorized), Invalid / expired To             ken

Why can I get this error and what can I fix it?

I tried to update my keys several times. I even uninstalled my application and created new ones several times, but I still keep getting this error.

In the "Application Settings" section, my access level is set to read and write:

Application Page Screenshot

I am not behind a firewall.

, Twython , , , , .