code: def get_access_token(self, verifier=None):
          
          try:
              url = self._get_oauth_url('access_token')
              self.oauth = OAuth1Session(self.consumer_key,
                                         client_secret=self.consumer_secret,
                                         resource_owner_key=self.request_token['oauth_token'],
                                         resource_owner_secret=self.request_token['oauth_token_secret'],
                                         verifier=verifier, callback_uri=self.callback)
              resp = self.oauth.fetch_access_token(url)
              self.access_token = resp['oauth_token']
              self.access_token_secret = resp['oauth_token_secret']
              return self.access_token, self.access_token_secret
          except Exception as e:
              raise TweepError(e)
docs: After user has authorized the request token, get access token
          with user supplied verifier.