code: def fetch_access_token(self, url, verifier=None, **request_kwargs):
          
          if verifier:
              self._client.client.verifier = verifier
          if not getattr(self._client.client, "verifier", None):
              raise VerifierMissing("No client verifier has been set.")
          token = self._fetch_token(url, **request_kwargs)
          log.debug("Resetting verifier attribute, should not be used anymore.")
          self._client.client.verifier = None
          return token
docs: Fetch an access token.
  
          This is the final step in the OAuth 1 workflow. An access token is
          obtained using all previously obtained credentials, including the
          verifier from the authorization step.
  
          Note that a previously set verifier will be reset for your
          convenience, or else signature creation will be incorrect on
          consecutive requests.
  
          >>> access_token_url = 'https://api.twitter.com/oauth/access_token'
          >>> redirect_response = 'https://127.0.0.1/callback?oauth_token=kjerht2309uf&oauth_token_secret=lsdajfh923874&oauth_verifier=w34o8967345'
          >>> oauth_session = OAuth1Session('client-key', client_secret='secret')
          >>> oauth_session.parse_authorization_response(redirect_response)
          {
              'oauth_token: 'kjerht2309u',
              'oauth_token_secret: 'lsdajfh923874',
              'oauth_verifier: 'w34o8967345',
          }
          >>> oauth_session.fetch_access_token(access_token_url)
          {
              'oauth_token': 'sdf0o9823sjdfsdf',
              'oauth_token_secret': '2kjshdfp92i34asdasd',
          }